PokemonGameEditor VS pokeemerald-speedchoice

this would be super easy to do, even if you don't want to get into the newer more complicated stuff like decomp, there are plenty of older tools out there like Pokémon Game Editor and Gen III Rom Hacking Suite that would work just fine if all you're doing is changing names of Pokémon, moves, items, trainers, etc.

Download Page Link: https://github.com/Gamer2020/PokemonGameEditor/releases

editing pokemon data should also be fairly simple depending on what you want to do. for this i recommend pokemon game editor. it has a variety of functions, but you can load your rom and pick the pokemon editor. pick the species you want to edit in the top-left. the species' catch rate is in the base stats tab, next to its other stats like HP and attack and so on. you didn't mention wanting to change evolutions, but that should also be really easy, and it's a popular edit in case you want to get ride of trade evolutions. just go to the evolution data tab and change "condition" from "trade" to something else like "level".

To get the evolve every level option you need the speedchoice version of an Emerald rom. You can patch a standard rom to speedchoice or find a pre-patched rom elsewhere.

pokeemerald-speedchoice/INSTALL.md at master · ProjectRevoTPP/pokeemerald-speedchoice · GitHub

there's also a pokered speedchoice if you're interested, as well as pokeemerald tho it looks like you'll have to build that one yourself.