I personally currently am studying further maths and maths alongside each other and am on course to an A* in normal maths according to my teachers but while I'm doing normal maths which I'm not finding too difficult but the further maths I'm doing right now is pretty tough imo, my exam board does pure maths, mechanics and statistics.
Pure maths - it's alright, wasn't too hard for me
Mechanics - haven't done it yet
Statistics - even though I've done like 5 topics in it, it's pretty tough for me and am only getting Bs and occasional As in homeworks...
Do it if you want to though, your choice.
btw the things I just wrote about my further maths course is CCEA (which you probably don't know since no one knows CCEA)