Seems like you're focused on the top end of the ecosystem. How about plant life, microflora, and microfauna? I've had good success building up layer by layer for self-sustaining no-filter setups.

Had danios and gouramis in the same tank. Will not do again. The danios are quite active. The gouramis are not. They tend to conflict. I prefer my tank to be peaceful rather than full of pieces.

Amanio shrimp are great, but know their offspring will not survive. They are magical to see them sprout, but will only last a day or so (the offspring need salted water to survive).

Good luck. Hope you'll get joy 😊

It can be an expensive hobby.

If you're trying to go cheap, you can save money by buying certain soils and sands from Home Depot (or your local equivalent) instead of substrate. Do your research on that before buying. You might be able to buy plant lights here, too, if you're okay with how they look or can make them look good.

Add plants. They look nice, provide hiding spots and naturally filter the water. Shrimp and snails add to the ecosystem. Some people will plant a pothos in the HOB filter for added water filtration and for the aesthetics.

Most of all, know what you want your tank to be before you get started. "Measure twice, cut once" and all that. I usually ignored the planning part and ended up spending a lot more when I changed from wanting a low-tech tank to wanting a hi-tech tank mid-stream.

There are games on Steam that help you design an aquarium (or just use paper and pencil). Dedicated fish stores sometimes have plexiglass sandboxes where you can experiment with hardscape placement.

Edit: I've had multiple peaceful gouramis before, but my most recent one (Blue Dwarf) ending up killing everything else in the tank.

Have patience. Choose plants and wildlife to suit your water supply.