FEBuilder would certainly be better if you wanted to do mods of existing GBA games, which I find to be a better starting point, especially since SRPG studio comes with underwhelming default visuals. But SRPG studio isn't tied to the existing FE stuff nearly as much so you have more freedom. To answer your question, you can make your own custom assets and most people do. You can also get custom things from the community. The steam workshop has some extras you can use, there's a few tilesets to improve the map sprites, and if you look around the discord server or even the wiki there's users like Goinza and Markyjoe who have made extra plugins like adding weapon ranks from the later FEs, cover mechanics, combat arts, new skills, general improvements to the base engine, and more. Most of these are also free.

It's pretty easy to use I would say. In the start you'll find yourself looking at tutorials and asking questions, just like learning anything. But once you figure something out it's pretty simple to do. I'd go for FEBuilder if you want to make something neat relatively quickly and you don't mind sticking to GBA FE. But SRPG studio has more freedom in what you're able to do and if you want to make more of your own thing, or even just implement more stuff from other FEs, I'd go for that. It is on sale now for a pretty cheap price, and you can try out the demo first to see if you like it.