Hey there,
I was in a very similar situation- regarding eligibility for UMEP, yes you would still be able to have it count as a 5th/6th subject depending on how well you do on it and other subjects- but do note that one of your subjects will have to be discounted from the top 4 and bottom 2 calculations (be it UMEP or another subject).
In my year, you could still do UMEP Physics without having done 3/4 physics, but you had to either HD the NQE (Physics Olympiad) or do a placement test.
Difficulty wise, it does take some more thought than a VCE subject, and be prepared to put in work for some of the more difficult topics, but workload wise, you won't have to do things like prepare for SACs (since assessment is basically the mid and end of year exams) or churn out as many practice exams so it's very manageable (I did 5 VCE subjects in Year 12).
Overall, UMEP Physics was very fun for me, but do have a think about whether you personally believe it is worth the time and effort to do a 7th subject.