heyy i do these, just not FM, honestly i find them not too bad, theyr're tough for sure, but it's better as you go through it since the content adds onto itself quite a lot in each subject which you need to makes sure you fully understand the basics but after that it gets easier
they also give a nice open option when it comes to choosing uni courses - i decided on engineering in the end but there is so much variety in the courses.
as for content wise, the jump is pretty big so you need to be aware of that but the subjects within themselves have a a fair bit of overlap, particularly maths (mechanics) and physics, and also dependant on your FM option modules I know further mechanics goes into a lot of physics. I find maths and physics overlap to be quite nice since the maths they always say the further kinematics/mechanics is the 'harder it' but you cover it earlier in physics so its almost just revision sessions in a sense/further practise rather than learning more content. There are minor chem and physics overlaps in thermodynamics/thermal physics but not that much
honestly the combo is manageable as long as you make sure you actively work and revisit content fro the minute you 'finish' a topic doing past paper qu's and familiarising yourself with what questions are asked, i like using PMT moslty as they have 'cheat sheets' with the basic info and a lot of resources for all 3 subjects (not sure abt FM sorry)
but also for chem/physics i'm not sure if every school/colege has it, but my sixthform we have acess to kerboodle which has the textbook on that you can use too and so many end of topic resources, and content glossaries > the ohysics one particularly has a Yr1, Yr2, and full list with ALL the laws and all the particular wording you need which is super useful as they love definitions in the exams so the full wording is great.
My key piece of advice is making sure in terms of revision is looking at the wording of questions since they ALWAYS try to catch you out.
hopefully this is some help, goodluck with everything