I think you are starting off from the perfect direction, being a forward-engineer first, and then a reverse-engineer.
The community around Frida is a a) a bit small and b) a bit unorganized/shadowy. You cannot find that many resources, atleast I have not found them.
I would suggest you to use Objection, explore an app, enumerate the classes with android hooking list classes or android hooking search classes, then dynamically watch and unwatch them. That is the quickest way to start, when you start developing your own scripts you can always check out code at https://codeshare.frida.re/.
For everything else join the Frida Telegram chat, most knowledge sits there, I am also there feel free to reach out to @altayakkus
Oh and btw, I would start with Android, even though iOS is fun too, and I would really really suggest getting a rooted phone/emulator. For the Android Studio Emulator you can use rootAVD (GitHub), just install Magisk Frida.
Installing the Frida gadget into APKs is a mess which you wont miss when you go root
