I thought about using RPG Maker, but as this is supposed to be a proper clone and not a fan game, I can't use Pokemon Essentials, because its license stipulates that games developed with it, even if they don't use any of the assets that come with the kit, can't be used for commercial use.
Is that really a problem, though? It sounds like you haven't done any game development or coding before, so I wouldn't get hung up on trying to sell something. Focus on just completing something first. Why not use Pokemon Essentials to create a small game, release it for free, and consider it a prototype and learning experience? You can get the core mechanics you want from the plugin, maybe learn some things about how it works, and get some practice with basic scripting on whatever's left. It's worth it to start small and work your way up.
I see that Pokemon Essentials only supports RPG Maker XP. XP is pretty old at this point, but it looks like it goes on steep discount pretty often and even has a free trial, so it's probably a decent cheap way to get started. If you are worried about getting frustrated and giving up, I think you should try the easy low-cost option first.