If a userscript is sufficient, it would be very simple. I’ve never written a browser extension, but it would definitely be more tedious because you have to get into writing the configuration and interface files for your add-on, in addition to programming the actual code. Although you can implement toolbar buttons etc., which is nice - but also not cross-platform! User-scripts are, and they’re also easier to write. They also allow you to add menu items, but you can’t have a dedicated icon and whatnot. It’s definitely not as pretty. Whatever you choose, Also, the community is here to help!
Anyway, the actual code logic behind your idea is pretty simple to implement.
My kanji highlighter script for example can open a new tab on WaniKani (or Jisho, if the kanji is not on WK), for all the kanji in the selected text, and that part is just a few lines of code. It’s basically like this:
For each character in the selection: Is a kanji? If so, open tab “Jisho.org: Japanese Dictionary”
Character code conversion would also probably be trivial and I guess it’s a matter of googling for 2 minutes.
genetickanji might be a problem since it uses some kind of page ID for each kanji, but it would be possible to parse the website and build a conversion table for kanji <-> URL. Could be a fun challenge for you as a beginner. 
[The basic idea is that you would download the HTML code for the site’s kanji index, and then either use an advanced text editor, or programming skills to parse out every kanji and its URL. You would then turn this list of kanji->URL relations into a format that Javascript understands, and put it into your source code. Then you could generate any URL for every kanji on that site]
What you’re trying to do doesn’t really involve any abstract concepts that are hard to grasp for beginners, and JavaScript is relatively easy to pick up (no need to worry about computer internals!), so you won’t have a hard time, I think.
It’s reaaally late here and I need to go sleep, so hopefully I didn’t miss anything.