I personally find 3&4 Specialist Maths very similar to 1&2, just minus some of the more difficult topics (sequences and series, proofs, number theory etc). The subject really doesn't deserve its bad reputation as being incredibly hard
And UMEP Maths is literally the greatest subject ever! Highly recommend to give it a shot. I'll write a full guide on why every single mathematically inclined student should select UMEP Maths as soon as I'm done with exams, but it is genuinely less than 10% of specialist maths workload and 5% of methods. (However, if you're doing Methods as well in Y12, 3 maths subjects might be a too much, but if you're passionate / enjoy maths it should be the right choice)