How much experience do you have keeping a planted aquarium? If you're a beginner, I highly recommend watching and reading as many videos about keeping a planted tank. Being prepared makes this so much easier.
Danios are a good choice for cold water. White cloud minnows are also really easy. For tetras, Buenos Aires and blood fin tetras live in cooler waters. If you have the experience, the celestial pearl danio is really beautiful. Cool water Cory's include peppered and panda.
For a 20 gallon, I recommend keeping 1 type of schooling fish and 1 type of cory as a 20 is not big enough to support multiple schools.
Neocardina shrimps like cherry shrimp are the easiest shrimp. Depending on how densely planted it is, they might breed.
Low tech plants include anubias, java fern, moss, wisteria, hygrophilia, crypts.
You have a wide range of filters to choose from.
You can do a simple sponge filter if you have a densely planted tank. It also has the benefit of protecting shrimp.
HOB filters (I recommend aquaclear) clean better than sponges, and you get space to put media in (aquaclear provides the most space). With a pre filter shrimp can be protected.
Canister filter. Most expensive but imo it's a HOB but better in every way.
For light, current usa, finnex, and fluval make good lights. They all have a timer to make things easier. You need to look through them and choose one that best fits your tank.
For substrate, for a low tech imo there is no need for a planted tank substrate. Regular sand and root tabs will work fine.
I hope this helps and good luck!