Good idea if your school will allow it. I know people who have started out with 6 AS levels, and then dropped down later. To start with 4 AS levels in Year 12 is very common, 5 isn't very many more.
If you are truly good at French then it shouldn't be too much work, and if you aren't actually fluent/native it will be good to have the A level as proof of your ability. Further Maths and Maths are the same, it's just maths, but more of it, so they work well together. Physics will have some overlap with the Mechanics modules of maths too so it isn't really 5 separate subjects at all.
I would suggest doing only 4 A2s or possibly 3 if you need to in order to really get the best grades but starting with 5 AS subjects can't really hurt, not sure what all the negativity is about here. Just don't be afraid to drop one if your grades aren't as good as they ought to be. I would suggest dropping Further Maths after AS because once you have done FP1 and all the Ms there's not much need to do any more, BUT check whether your desired universities want Further Maths A2 - most do not require it.