Welcome Rhymaestro,
if you want to modify Ragnarok to fit to your ideas, you have several options and things to learn.
The simplest way to change the world itself is using scripting, it lets you do a lot of stuff, but based on how deep you want your changes to be, you will reach its limits relatively fast.
This brings us to source modification, for which you will need C++, it will allow you far more agency but is also more complicated and harder to learn.
Custom jobs for example would be a thing that you would implement via source modification.
You will also need to grasp at least a bit of how the client works, where it gets the graphics and information from, so you can extend or modify things to your liking.
Taking your custom job idea, you would need to change client files for the skill descriptions, for example.
Many things you want to achieve will have been done in one way or another already. So searching the forum or checking the download section for scripts or source mods you can use as a base or learn from might be useful.
You can also check the rAthena wiki at https://github.com/rathena/rathena/wiki which has a lot of guides on how to install rAthena and connect to it or how to achieve certain things you may want to do like adding custom items or mobs.
Another helpful resource is the doc directory in the rAthena folder. You can find an online copy here: https://github.com/rathena/rathena/tree/master/doc it provides a lot of information regarding the structure of rAthena to make it clearer how it works.
I think especially helpful are the script_commands.txt, which is a reference of all scripting commands rAthena provides and the source_doc.txt which explains how the servers communicate with each other, how the code is organized and what a lot of the acronyms often seen in the source code mean.
Tool and resource wise, I can recommend you the tools from:
Ai4Rei: https://nn.ai4rei.net/ (Check yourself which tools you might need)
Tokei: https://rathena.org/board/files/file/2766-grf-editor/ (GRF Editor)
llchrisll: https://github.com/llchrisll/ROenglishRE (English translation of the client)
4144: https://nemo.herc.ws/downloads/ (A good source for full clients, unpacked client exes, and you can get nemo patcher from there to patch your client)
Feel free to ask any questions you have.
I wish you the best and look forward to seeing how your project progresses.