PokemonGameEditor VS PokEditor

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".

PPRE should be avoided if that's possible. i believe pokeditor is the recommended tool for modifying things like evolution data.

Now there is one tool I didn't mention not because it's bad but I think it's the wrong time to start using it https://github.com/turtleisaac/PokEditor

pokeditor takes pokemon data from the rom and constructs it as a spreadsheet, then re-inserts that data from the spreadsheet after you've made changes. this makes it compatible with existing hacks. it's a command-line tool without a user-friendly interface but it if you're willing to put up with that it is the recommended option for gen 4. it also seems to have gen 5 support but that may be buggy.

editing pokemon base stats and stuff can be done thru pokeditor. it says it has support for gen 5, but there are no gen 5 templates, so... read the instructions closely and use it with caution. there might be other editors for pokemon data out there too.