_sophiestudies_
I'll try to spill some tea on the units/majors since I have friends who did them. Keep in mind that this is 2nd hand info as I've only taken units in biomed (my previous degree), chem (my previous electives and my current major) and 1 MTH unit (MTH1020).
With the BCH minor, I heard that the 2nd year BCH units are baits for the 3rd year ones. Level 2 BCH are taught very well and are very easy (I've seen the content and it's easier than the biochem we covered in 1st year biomed), but BCH3XXX units are all very difficult with really high workloads.
I heard that IMM and PHA are hard, although they're well run. There was one year where IMM had an exam that didn't allow you to come back to questions you've skipped, but that was just an one-off thing during remote learning.
PHY2011 is a well-known WAM booster, the unit coordinator (Ramesh) has achieved legendary status among the Monash sci cohort for knowing every student in the annual 700 strong PHY2011 cohort by name. His assessments are all open book. I've seen people get 96-98 HDs in that unit with very minimal effort.
DEV is a great major from everyone doing it. It wasn't my cup of tea because I hate flashcarding absolutely every detail and rote learning. The 2nd year DEV units are well run with a similar reputation to BCH2XXX, although they made both DEV2011 + 2022 in recent years harder apparently so they're not as WAM-boosty as before. Everyone I know who did DEV also loved the 3rd year units, but they're very difficult while being equally rewarding at the same time
I'm not very familiar with the school of biological science ie with conservation bio, I only know about biomed-related electives.
However since you indicated that you're looking at postgrad, definitely consider doing a 3990 unit (research project unit). Even though I never planned to go into research, doing a 3990 unit really improved my oral and written communication skills and I was able to research + reference much faster in later units.
Also, please do your research on the costs-benefit of postgrad. My CHM3941 TA last sem told us to "definitely do 3990 units, definitely do honours, but as soon as your supervisor in honours starts sweet-talking to you about starting your PhD, run for the door". He was in his final months of a PhD and told us PhD kids are essentially heavily exploited as slave labour, so you may want to take into account the anecdotes of different PhD students before you make a decision