I'd recommend you play some games before you dive into designing. That way you can get a feel for themes and mechanics and what works and what you like.
The good news is that you don't need anyone to play with. There are plenty of TTRPG systems designed for solo, or you can always play more traditional systems solo using a GM emulator or similar.
Solo TTRPGs tend to come in a couple of general flavors - journaling games, and more "traditional" RPGs. And each of those come in various complexity levels.
Here are a few of various flavors that can help get you started. Check em out to get a feel for what's out there, what other folks are doing, and what works for you. Lots of folks with start with tweaks, extensions, or hacks of existing games.
The story quests should be pointing you in a couple of different directions by now. You should be equipped enough to follow either of those at this point.
The overland route specifically will net you plenty of shrines along the way to continue pumping up your hearts - there are a few caves too and those are always full of surprises.
Just be smart about what fights you take on along the way. Two things about fights in this game:
The game doesn't punish you for dying, unless you're overly sensitive to loading screens. Try different strategies, get a feel for the advanced combat mechanics. You'll probably end up at a few different combat training shrines if you're following the early game quests and that'll help you out if you're unfamiliar with some of the more advanced techniques.