Smaller Axial Tilt

Thicker Atmosphere

Shorter Days

Don't let it get too hot.

That said, when you increase the atmosphere and coriolis effect, the temperature does not just get more stable, it gets hotter and windier too. One way or another, this planet will be more prone to hurricanes than Earth is, but by keeping it relatively cool, you can control how strong the hurricanes can get. Hurricanes are powered by heat; so, as long as you keep the oceans around a modest room temperature, you don't have to worry about any high category destroying everything in thier path. To do this, you will want the planet to be farther from the sun relative to its luminosity compared to Earth.

How to solve for a planet that is 100% true tropical zone.

Smaller Axial Tilt

Thicker Atmosphere

Shorter Days

Don't let it get too hot.

That said, when you increase the atmosphere and coriolis effect, the temperature does not just get more stable, it gets hotter and windier too. One way or another, this planet will be more prone to hurricanes than Earth is, but by keeping it relatively cool, you can control how strong the hurricanes can get. Hurricanes are powered by heat; so, as long as you keep the oceans around a modest room temperature, you don't have to worry about any high category destroying everything in thier path. To do this, you will want the planet to be farther from the sun relative to its luminosity compared to Earth.

How to make a planet as climatically homogenous as possible.

While a zero axial tilt gives each part of the planet a very stable climate, and the other stuff will help keep the whole planet about the same temperature, the poles may not get enough direct sunlight for good photosynthesis. There is another solution where your planet will still still have seasonal zones, but the whole plant will always get enough light to support life.

Binary Star System

Imagine a binary star system with one small dim star like a brown dwarf (StarA) and a much brighter main sequence star (StarB). Next place an Earth like world in a small short orbit around StarB such that orbital distances give both stars have about 1/2 of our sun's apparent luminosity. This is possible because a star's mass to luminosity ratio is not 1:1; so you can be gravitationally bound to just the smaller star while receiving equal amounts of light.

Because you are so close to the smaller star, you will also have a very short StarA year. Make this about 3 days so that you can give it as short of a year as possible without the day/night cycles of the two stars syncing up in counter productive ways. Next give the planet about a 45 degree axial tilt. This way the North pole gets StarA's light for about 1.5 days, and then the South pole gets it for 1.5 days. Not long enough to create a proper season, but it is long enough so that half the planet's heat and light comes from what hemisphere you are on and the rest of your heat and light will come from StarB's Day/Night cycle. StarA makes sure that each latitudes gets enough heat and light on a regular basis while StarB makes sure that each longitude gets enough heat and light on a regular basis.

Your planet will still experience latitudinal seasons like Earth does, so, it's technically not a 100% true tropical zone world, but StarA will significantly offset the the severity of the winter/summer cycle so instead of your planet having an average 50°F degree seasonal variance in the temperate zone like Earth does, it will be more like a 25°F degree seasonal variance.

Combine this with a thicker atmosphere and faster spin and you should be able to get the seasonal variance down to less than 10°F making it more or less unnoticeable.

The one thing of note in this setup is that far more of your light would have to come from infrared than on Earth; so, your native life forms may be adapted to see and photosynthesis in different parts of the spectrum than earth based life.

Smaller Axial Tilt

Earth's tilted axis causes the polar regions to suffer from a year long day/night cycle which would be problematic to life even if it was relatively warm. Also, a tropical zone by definition has no winter/summer cycle. So, if the Axial tilt were essentially zero, it would not matter how far north or south you go: Day and Night would stay about 12 hours each, and there would be no noticeable seasons. Definitionally speaking, a world with no axial tilt is all Tropical Zone, even if some parts are cooler than others.

Thicker Atmosphere

But I know you are looking for more than just Tropical seasonal patterns; so, the next thing is a thicker atmosphere. In general, the thicker the air is, the more thermal energy gets moved around on the surface. This will help to stabilize the temperature between the equator and the pools because there will be stronger thicker wind currents circulating hot and cold air between the regions. That said, when you do this the temperature does not just get more stable, it gets hotter too. So, you will want the planet to be farther from the sun relative to its luminosity compared to Earth to keep it from evolving into a Venusian world.

Shorter Days

While more air helps carry more energy for how much it moves, the force that actually gets these currents going is the Coriolis Effect. This is the principle that causes air currents to move in large circles around the planet bringing air between the poles and the equator. The faster your planet spins, the faster and stronger these currents will be that help mix the cold northern air with the hotter equatorial air.

Smaller Axial Tilt

Earth's tilted axis causes the polar regions to suffer from a year long day/night cycle which would be problematic to life even if it was relatively warm. Also, a tropical zone by definition has no winter/summer cycle. So, if the Axial tilt were essentially zero, it would not matter how far north or south you go: Day and Night would stay about 12 hours each, and there would be no noticeable seasons. Definitionally speaking, a world with no axial tilt is all Tropical Zone, even if some parts are cooler than others.

Thicker Atmosphere

But I know you are looking for more than just Tropical seasonal patterns; so, the next thing is a thicker atmosphere. In general, the thicker the air is, the more thermal energy gets moved around on the surface. This will help to stabilize the temperature between the equator and the pools because there will be stronger thicker wind currents circulating hot and cold air between the regions.

Shorter Days

While more air helps carry more energy for how much it moves, the force that actually gets these currents going is the Coriolis Effect. This is the principle that causes air currents to move in large circles around the planet bringing air between the poles and the equator. The faster your planet spins, the faster and stronger these currents will be that help mix the cold northern air with the hotter equatorial air.

Don't let it get too hot.

That said, when you increase the atmosphere and coriolis effect, the temperature does not just get more stable, it gets hotter and windier too. One way or another, this planet will be more prone to hurricanes than Earth is, but by keeping it relatively cool, you can control how strong the hurricanes can get. Hurricanes are powered by heat; so, as long as you keep the oceans around a modest room temperature, you don't have to worry about any high category destroying everything in thier path. To do this, you will want the planet to be farther from the sun relative to its luminosity compared to Earth.