I can't really give advice as per your questions, but there is one thing that leaves me a little uneasy: you have no safety net for your primary four.
The way the ATAR works, if you're unaware, is that your top 4 subject study scores get added together to make an aggregate. If you have a fifth or sixth subject, 10% of those study scores get added to your aggregate. The aggregate gets turned into an ATAR later using the VCAA magic wand based on the aggregate everyone else gets. What's important here, is that the higher your aggregate, the higher your ATAR.
So here's the thing - let's say you have five subjects, and can score really highly in all of them. You then get a 35, a 32, a 28, a 29, but then you bum out and only get a 20. That's okay - because the 20 is only going to contribute 10% anyway. But what if the subject that you get 29 in is your uni subject?
Uni enhancement subjects /can't count in your primary 4/. That means that you've effectively lost 9 aggregate points, which in 2019 would've been an ATAR of 66 instead of 72. This might sound fine considering you're only aiming for 60 anyway, but clearly-in ATARs change all the time
It's ultimately your choice, and if you're an A-A+ student, getting those study scores or higher might be super achievable and so this not being an issue - but having that fifth subject safety net is honestly really valuable, and tbh I'd do it just for the peace of mind