I can't answer all the questions. Your Aus PPL will stay valid, to use it, all you need is an Australian medical and a flight review of some sort. You won't lose that PPL. If you convert, you're not exchanging one for the other, you're using the PPL to get credits so to speak. Eg, when I moved over, I used my Aus ATPL to give me some flying credit for the JAA ATPL (which became the EASA ATPL, then I had to change it from the UK CAA to NL thanks to Brexit) and now hold both an EASA ATPL and UK CAA ATPL. My Aus ATPL is still there, just need an Aus medical and a flight check.
Flying on a PPL is relatively easy. Not a lot to it. If you go integrated, it'll just sit there all nice and quiet.
Have you looked at the CAA's website? The UK and EASA licences are not the same any more. I understand 2 sets of exams (which is possible...) and specific flight tests done in each jurisdiction. I saw somewhere on this site a very clever flight test involving Jersey and Caen/Le Torquet. Brilliantly clever.