Not sure if you have already, but have a look at the undergraduate fac of science handbook, particularly the unit of study table for each major (science undergraduate > Table A > subject areas > find the major you want and go to unit of study table)
This outlines the specific subjects you need for each major and any other requirements. in first year, a lot of majors will have some overlap. For example, for all of those majors, you'll need to do BIOL1007 - from molecules to ecosystems (or the advanced/SSP version of it). For your other 1st year subject for whichever major you choose you will likely do another biology unit or a chemistry unit. You get much more opportunity to specialise later on once you have the fundamentals down.
The good thing about this is that it means you can very easily change majors later on without being behind. Also, remember that you will have other subjects you can do in first year. Each semester (at least in first year for a B.sci) you will normally do 1 subject for your first major, 1 subject for your second major/minor, 1 core unit (maths or data sci. kinda stuff) and 1 elective subject. So, you could pick your majors, lets say immunology and pathology, and biology. But as an elective, you could also take chemistry 1A and be fully up to date if you wanted to change into say a microbiology major.
Also, im not sure how this works, but look into what happens when one subject is supposed to count for both of your majors - im not sure whether or not it can count twice and you do an extra elective, or if something else needs to happen.
I can't really help with the relative difficulties too much, but most majors have room to specialise and challenge yourself as much as you want.
Good luck, hope this helps
