The most cost-effective way to get lower gears is probably to change the rear cassette. 12-28 tooth 8s cassettes exist and are fairly cheap (I believe about US$20 online, possibly $35 or so from a shop, plus installation). This should lower your lowest available gear by about 19% (when calculating gearing in gear-inches, anyway).

You stated that your RD capacity is 29 teeth. Capacity considers both the difference teeth at the front, and the smallest to largest cog difference at the rear. You're currently using 52-42 = 10 teeth of capacity at the front. 28-12 teeth is 16 teeth. Summing those, you would use 26 teeth of capacity with a 12-28 cassette. You could likely go up to a 12-32 cassette, which would exceed the official capacity by 1 tooth. However, Shimano has been conservative in its capacity specifications, so you can likely get away with this. That cassette would produce something like 29% lower gearing in the biggest cog.

The trade-off is that the gaps between gears will be bigger. On group road rides, this is likely to be annoying. In that use case, you want to be able to produce your optimum cadence at the group's current speed. That type of riding does favor more closely-spaced cogs on the rear. On solo rides, this is a matter of individual preference, and it may be totally irrelevant to you.

Your crankset can accommodate a 39t or 38t inner chainring. This reduces your gearing by something like 5%. Inner rings are also cheap. Your front derailer can shift the resulting 14 tooth gap, although that affects rear derailer capacity also.

Further modifications to the bike are possible. You could upgrade to a more modern compact crankset with 50/34 chainrings, or even 48/32. However, this almost surely requires a new bottom bracket, unless you found a square taper compact crank that fits on the square taper spindle you have - many square taper cranks may need a different spindle length to achieve optimum chainline. You could search on Google if you desired; there may be some cheap options, but many of the ones you find may be expensive. You could even get a 10s or 11s group second hand, although it will be used. By this point, it would probably be more cost effective to get a new second hand bike. Changing the group either needs you to get the tools and to learn how to install it, or it requires you to pay a shop to do this, so it is an expensive option.

(Note: a previous version of this answer forgot that the OP specifically said that the max cog size of his RD was 27t. The answer has been modified.)

The most cost-effective way to get lower gears is probably to change just the rear cassette. 12-28 tooth 8s cassettes exist and are fairly cheap (I believe about US$20 online, possibly $35 or so from a shop, plus installation). . You stated that your max cog size is 27t. Shimano's specifications tend to be conservative, and a 28t rear cog is very likely to work. This should lower your lowest available gear by about 19% (when calculating gearing in gear-inches).

To get even lower gears, you could likely go up to a 12-32 cassette, but you would also need to change the rear derailer. That cassette would produce something like 29% lower gearing in the biggest cog. A 12-30 cassette also appears to exist. However, both exceed the maximum sprocket size by quite a bit, and they also exceed your rear derailer capacity. You could look for a rear derailer compatible with your shifters that has the correct maximum cog size and enough capacity. For the record, your crankset uses 52-42 = 10t of capacity. A 12-32 cassette would use 32-12 = 20t of capacity. That configuration uses a total of 30t, or 34t if you change to a 39t inner ring. You might also explore an item like Wolf Tooth's Roadlink to extend the capacity of your current RD, but I'm not sure how this item works with 8s drivetrains.

The trade-off with all these cassettes is that the gaps between gears will be bigger. On group road rides, this is likely to be annoying. In that use case, you want to be able to produce your optimum cadence at the group's current speed. That type of riding does favor more closely-spaced cogs on the rear. On solo rides, this is a matter of individual preference, and it may be totally irrelevant to you.

Your crankset can accommodate a 39t or 38t inner chainring. This reduces your gearing by something like 5%. Inner rings are also cheap. Your front derailer can shift the resulting 14 tooth gap, although that affects rear derailer capacity also. You physically can't mount an inner ring smaller than 38t on this crankset.

Further modifications to the bike are possible. You could upgrade to a more modern compact crankset with 50/34 chainrings, or even 48/32. However, this almost surely requires a new bottom bracket, unless you found a square taper compact crank that fits on the square taper spindle you have - many square taper cranks may need a different spindle length to achieve optimum chainline. You could search on Google if you desired; there may be some cheap options, but many of the ones you find may be expensive. You could even get a 10s or 11s group second hand, although it will be used. Changing the group either needs you to get the tools and to learn how to install it, or it requires you to pay a shop to do this, so it is a relatively expensive option. By this point, it would probably be more cost effective to get a new second hand bike. The second-hand bike market has quite low prices, especially for rim brake bikes. This is because the latter have been superseded by disc brake bikes. However, and this is coming from someone who has one of each, rim brakes are more than adequate for road bikes in almost all conditions.

Last, I hope you come to enjoy cycling more and more. As you ride more, you will get stronger. Even if a 28t big cog is a little overgeared for you right now, chances are it will become less and less so.