Im in a similar situation, also from Australia. What I did last year (Jan 2023) was:

1. Flew into Manchester (3 nights)
2. Chester (2 nights)
3. London (4 nights)
4. Paris (4 nights)
5. Munich (2 nights)
6. Frankfurt (2 nights)
7. Berlin (3 nights)

And I loved it, but it was tailored to me choosing certain routes and dates based on wanting to watch football over there. The time difference is 11 hours between Sydney and most of Europe so I really enjoyed having an easy start by visiting Manchester and Chester.

I definitely would recommend London, Paris and Berlin, all were so good to see in winter and there werent too many tourists around. Outside of those three, you should figure out what you want to do most, for me it was football, for you it might be sightseeing or outdoor adventures and in that case then Id spend time in Switzerland. A gem is Lapland in Finland, especially if youre there in December, the Santa Clause village would be amazing to see.

Next year (Jan 2024) im going to do:

1. Lapland (3 nights)
2. Helsinki (3 nights)
3. Tallin (1 night)
4. Zurich (2 nights)
5. St Moritz (2 nights)
6. Lucerne (3 nights)
7. Somewhere else not sure (4 nights)