A Dyson swarm around the Sun would probably be built with orbits at about 1 Astronomical Unit or AU from the Sun, so it would have a diameter of about 2 AU.
A light year is 63,241.077 AU and a parsec is 206,264.806247096 AU.
https://en.wikipedia.org/wiki/Light-year#Definitions
https://en.wikipedia.org/wiki/Parsec
The nearest star system to the Solar System at the present time, Alpha Centauri, is more than four light years, and more than one parsec, away.
Here is a link to a list of stars which are calculated to have passed with 5 light years of the Sun in the last few million years, or to pass within 5 light years of the Sun in the next few million years.
https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs#Distant_future_and_past_encounters
That shows that sometimes stars will approach each other closer than one parsec, and even on rare occasions closer than one light year.
So imagine that your agrarian world happens to be one light year from its nearest neighbor for a period of several thousand years, a rare occurance, and that nearest neighbor star is one with a Dyson Swarm.
At a distance of 1 light year, or 63,241.077 AU, a full circle will have a circumference of 397,355.07 AU. With 360 degress in a circle, at a distance of 1 light year a degree of arc will be 1,103.764 AU wide, an arc minute will be 18.396066 AU, and an arc second will be 0.3066011 AU wide.
So at a distance of 1 light year, a Dyson swarm 1 AU wide will have an angular radius of 3.2615669 arc seconds and an angular diameter of 6.4331338 ar seconds.
If you character is a human, they won't be able to see the Dyson swarm as a little blob.
The maximum angular resolution of the human eye is 28 arc seconds or 0.47 arc minutes,[19] this gives an angular resolution of 0.008 degrees, and at a distance of 1 km corresponds to 136 mm. This is equal to 0.94 arc minutes per line pair (one white and one black line), or 0.016 degrees. For a pixel pair (one white and one black pixel) this gives a pixel density of 128 pixels per degree (PPD).
https://en.wikipedia.org/wiki/Visual_acuity#Physiology
So the smallest angular diameter a human can see is about 4.35 times as large as the angular diameter of a Dyson swarm with a radius of 1 AU at a distance of 1 light year.
I may point out that the angular diameter of R Doradus, the star with the largest measured angular diamter, is 0.057 plus or minus 0.005 arc seconds, or 0.052 to 0.062 arc seconds.
https://en.wikipedia.org/wiki/R_Doradus
But if all the stars have angular diameters as seen from Earth much smaller than what the human eye can resolve, how can humans see any stars in the sky?
We see stars as dimensionless points of light. Or we would, but the Earth's atmospheric motions make the stars twinkle and appear a lot wider than their actual angular diameters.
So don't worry, the observer will be able to see the visual band radiation of the stars with Dyson swarms as tiny points of light, just as well as he sees all the visual band radiaiton of the stars without Dyson spears. Everything will be fine.
Except that the purpose of a Dyson swarm is to trap all the radiation in all the various bands, including visible, of its star, and to use the energy from that radiation for useful purposes, and then emit the waste heat from those processes as infrared radiation.
So yur stargazer will not see any visible light from any of the stars which have Dyson swarms.
If the stargazer is human, that is. Maybe the planet's atmopshere is transparent for the main infrared radiation bands of Dyson swarms, and the natives of that planet can see light in those infrared bands, and thus see the infrared from the Dyson swarms. Presumably all the stars which don't have Dyson swarms will also emit a lot of radiation in infrared and also be visible to such an alien stargazer.
Of course some stars will have higher luminosities than others, and some will emitt higher percenages of their radiation in the infrared than others, and some stars will be much closer than others. Thus the stars will appear to have greatly different apparent brightnesses in infrared light, just as stars seen from Earth have greatly different apparent brightnesses in visible light.
And I don't know how to figure out whether the stars with Dyson swarms will appear different enough in the infrared from stars without Dyson swarms, for an infrared seeing native to tell which was which, since there would be visible diferences between various stars anyway.
Or maybe the nonhuman native of the agrarian planet will have two (or more) sets of eyes, some to see visible light and some to see infrared light. So the native could try looking at the sky with both sets open at once, and with only the visible band eyes open and the others shut, and with the visible band eyes shut and only the infrared eyes open. I think that the sky could look a lot different with different eyes open.
I note that some stars have disc of dust around them, which are heated up by the radiation from the stars and thus emit a lot of infrared light from a much wider area than the diameters of the stars. Could such dust discs have angular diameters large enough to be seen by the naked eye?
The excess infrared radiation from the star Vega is calculated to come from a disc with an outer radius of 120 AU, and thus an outer diameter of 240 AU. As we should all know (since it was first measured over 180 years ago) Vega is about 25.04 plus or minus 0.07 light years, or 7.68 plus or mius 0.02 parsecs, from Earth.
https://en.wikipedia.org/wiki/Vega
A parsec is defined as 206,264.806247096 AU because at that distance an object 1 AU wide would have an angular diameter of 1 arc second. Thus parsec is short for "parallax second".
So if Vega is about 7.68 parsecs from Earth, the angular diameter of Vega's dust disc should be about 240 arc seconds divided by about 7.68, or 31.25 arc seconds, a little above the 28 arc second minimum to be resoved by human eyes.
The star Fomalhaut is about 25.13 light years or 7.70 parsecs from Earth. It has three rings of dust around it. The outermost ring has a radius of 133 AU and a diameter of 266 AU. at a distance of 7.7 parsecs it will have an angular diameter of 34.54 arc seconds, a little wider than Vega's disc.
https://en.wikipedia.org/wiki/Fomalhaut
The star Epsilon Eridani is about 10.475 light years or about 3.212 parsecs from Earth. It hos two suspected planets, two asteroid belts, and a dust disc with a radius of about 35-100 AU and a diameter of about 200 AU. Thus the disc has an angular diameter of about 62.2665 arc seconds.
So if your agrarian planet is in a stellar neighborhood similar to ours, a few stars should have dust discs large and near enough to just barely resolove with the naked human eye. However, those discs would be emitting infrared light, not visible light. Aliens who could see see infrared light would need to have larger eyes than humans do to resolve infrared images as acutely as humans resolve visual light images, let alone better than humans do.
I also don't know how brightly those dust discs glow in the infrared. It is possible that they are too dim in the infrared to be seen by infrared seeing aliens.
Or it is possible that your stargazer might be a human, or an alien with vision similar to humans. Thus they would only be able to see the stars without Dyson swarms, and the stars with Dyson swarms would be invisible to your stargazer.
Unless they use some sort of night vision equipement, which senses infrared light, and so can see the stars with Dyson swarms with that equipement.
Switching between their unaided vision and the night vision equipement would show somewhat different sets of stars.
Or possibly they use binoculars or an amateur telescope, and switch the night vision equipement attached to the telescope on and off to get different images of the sky. Using a telescope would enlarge the dust discs around a few stars, perhaps making them visible as shapes through the telecope.
It seems that not all the planets in your fictional empire have Dyson swarms yet. If the agrarian planet is inside a complete dyson swarm no stars should be visible, and if the swarm is still being constructed a lot of stars would probably all readeady be hidden by the swarm. And of course the crops and all other plants would die if the agrarian planet was outside the Dyson swarm.
