MUEP chemistry needs more commitment than MUEP maths because you have to do pracs/lab work and as a result have to go the place more than once a week and produce prac reports as well as assignments(im not sure about assignments), Also only some of the stuff is similar to VCE chem and it might be hard to learn new concepts, also the structure is not like VCE chem where you do analytical chem/biochem in Unit 3 and industrial chem in unit 4, Monash is more of a mixture with other stuff to. But if you are interested then go ahead, i dont do muep chem but ive heard its quite a handful.
However, for MUEP maths you only have to go once a week, (5 or 6 for the whole year) assignments is the only homework you have to do.
semester one is heavily based on VCE methods/spesh. Semester 2 little less emphasis and you learn new things. Its not as time consuming as MUEP chem (i do MUEP Maths), Personally i dont really enjoy it, im more of a chem person however my school spesh/meths teacher offered to help me whenever i needed it so yea.
Physics . . . this just depends on you, if you find physics hard this year then i heard year 12 physics is easier compared to year 12 chem. Physics is like a math subject workload you just have to constantly practice, and you got lessons more often since it is at school, compared to the MUEP subjects which are once a week with the exception of chem. (i chose to do MUEP maths instead of physics coz of free period awesomeness)