1. Religiously complete an exercise (if you're still learning the content) or a past paper every day. Most of the time (unless you're SUPER dedicated) you probably lose track of a task and procrastinate by doing something not related to your studies, BUT if you instead remind yourself that it'd be more beneficial to do some maths instead, you can then get back to working on the task for a little bit more. Also, if you treat the maths exercise/past paper as something you have to hand in (even if you don't have to) you will prioritise it. Also bear in mind you don't have to do a full exercise or paper. If you legitimately feel strong with a concept (especially the ones that aren't hard but just require heaps of expansions etc.) just gloss over it in a past paper or exercise (explained more at the end of this).
2. Not at this point. Again as a user above said, a LOT of people start 3U with 50s or even lower and then go on to achieving high marks in their HSC year. Mainly, it's because 3U maths is very foreign and different to most of Stage 4 and 5 maths. I'd also recommend getting on top of 2U theory (ahead of your class even - by working through the textbook) so that the fundamentals for 3U are strong. Also since you've been saying you struggle with 3d trig, make sure your 2d trig is near perfect and you understand all the ways to approach 2d trig questions.
Tip: 3d trig is normally just about splitting the image into heaps of triangles. Draw them out with labels and then match labels to each triangle that hits a certain side (hope this makes sense - maybe you already know this already)
3. Not at all. Kind of goes on from answer to Q2
4. Definitely consider it. I would again say don't base any of your year 12 decisions on most of your prelim stuff. Maybe yearlies, but just because it might be an indicator that your grasp on the topics isn't as strong as it should and would take too much energy to catch up on (definitely wouldn't be able to tell if this is the case at this stage though). I think if you follow tip in Q1, you will definitely be able to manage.
I would say that most of 4U isn't really that hard. Sure, the wide variety of questions and critical thinking necessary makes it hard to full mark a lot of questions but the fundamentals are not that hard to get (usually extensions [ik, sounds obvious] of 2U topics) - e.g. 'conics' (a 4U topic) is like using coordinate geometry with just shapes that have different general formulas.
Just one more thing. Make sure you're not just doing questions because they make you feel better about how you're doing. Not to say that you are, but I personally have many times done easy exercises because they make me feel like I know the content really well, and I neglect the hard stuff. Going to teachers or forums like these with hard questions you don't understand is the most important part of doing exercises and becoming better. It is good to strengthen your knowledge of topics to not do dumb mistakes, but not to the point where the stuff you find hard gets neglected. For example, definitely do a chapter on 3d trig if it's going to come up in your next test. If the first couple of exercises have stuff that you don't struggle with, for time's sake, keep looking until you get a question you don't know how to do and then come here or sticky note it for a teacher. Then, if you have free time you can do the other questions. Just prioritise the right topics and you'll be fine. Same thing applies with past papers (ONLY if you're doing them in untimed conditions and way before the test - otherwise do the whole paper if it's in timed conditions and you're going to mark it as if it were the real thing).
These are a few things I would have liked to known when I was doing prelim (I feel like it would have reduced my stress) and I hope it helps you out
