Congratulations @teepee on getting through!
Can you access a rehab oncology exercise programme as movement and strengthening does help? I'm not sure what is around in Perth - maybe some locals can help. Possibly Encore (YWCA) which is a pool-based programme for bc. If you have private cover, you may be covered for some programmes. Otherwise, you can access a number of physio programmes with a health care plan from your GP. I believe there's some mentioned on this site from the home page - also check out Pinc & Steel as they link with physios who work with oncology issues. Once you know what is best for you, then you should be able to work out what you can do on an ongoing basis. I'm no expert but I think the trick is to build up gradually, with help from someone who understands the possible issues, and go from there. Of course, walking is great exercise that probably won't get you into strife. You do need to be careful not to go too gung-ho with arm weights as you don't want to trigger lymphoedema - that's why it's good to have someone knowledgeable to guide you in the beginning. And you may be a bit weak after months of treatment.
Other things may be dragon-boating or things like that but I would suggest that you make sure you work on regaining some strength if you have lost it during treatment.
As for diet, I don't think much changes from what you probably know from before bc - as healthy as you can - watch the alcohol intake...and the sugar...and the convenience food. Easier said than done when life gets busy again and you're tired.
Re: supplements...there is a site that I can't remember that gives a lot of information about this (@romla). You probably should check with your medical team as to what you might need (Vitamin D, calcium, etc...) but, unless you're actually lacking in most things, taking supplements won't do much (and some "natural" things can have nasty side effects). Personally, I've even stopped taking a calcium supplement, with my onc's okay, due to the possible heart risks - I'm trying to keep the bone density up with extra calcium in my diet and weight bearing exercise.
I don't know what your home situation is but I think the most valuable thing to know is that you must be insistent with your family (and yourself) that you need to factor in non-negotiable time to do things for yourself. It can be hard because for many of us, our time is the first to be lost in the busyness of life. I did do Encore at the end of last year. I have my weights by the telly so that they're on hand for that moment of an evening where I have the slightest spark of energy. I have also, after much searching and planning (a bit of a logistical nightmare), designated at least one night per week where I go to the pool for an aqua session. For a change, everyone has to plan around me for that one. When the weather was better, I was driving down to the beach (an hour away) every 2 or 3 weeks to go paddleboarding early in the morning. I've learnt to work out options, then say this is what I'm going to do...does anyone have any questions. Then I do my best to stick to my guns. It also helps that, because I've got everyone on board, there's a certain expectation for myself that I'll follow through.
I've also found that some mindfulness meditation is helpful when you get overwhelmed. There's a lot of different apps out there to help. I use Insight Timer and have a number of different meditations bookmarked on my phone.
I hope that some of this helps.