I don’t consider Unreal fit for making source files (textures, meshes, animations, audio) and so on. You’ll be depending on its limited functionality and tied to UAssets while you could pick any external tool specialized to do the job correctly. Go with blender. It will suck in the first days or weeks so instead of smashing keyboards you learn the basic controls, layout, then follow the 4 part tutorial. Export mesh + skeleton to FBX and drag the FBX into the editor where it will turn into the assets you are used to. There are tutorials on all those steps on youtube.
If you were to modify assets inside the engine you would very quickly run into annoyances like I discussed here: