We did this itinerary Christmas/NYE 2018. Your itinerary is almost identical to the one we did. It is a wonderful cruise!
Perth is a lovely city. Plenty to do for a couple of nights. There is a lovely waterfront and great restaurants right in the city. Look to see if they will hold the holiday show in the park while you are there. We were there over the same dates as you, and it was really wonderful to be outside in the park with the holiday music. Also, try to tour the prison in Fremantle, where the ship docks.
Albany is a small town and easy to walk. We didn't do anything special other than wander the town and visit shops.
In Adelaide, you will want to take the train from the cruise port into the city. It was SCORCHING hot the day we were there. Plenty to do in the city. No shortage of food and shops. They also held our Azamazing Evening in Adelaide, which was a performance by a known Australian pop singer. It was held in a beautiful venue, but it was nothing amazing. Worth it just to see the venue to have a nice evening with other passengers.
Kangaroo Island was incredible! We booked an island tour through the ship. Fantastic guide and incredible wildlife. We saw plenty of koalas and kangaroos out in the wild. We were able to get fairly close to the koalas that were in the trees. We also toured the coast and some of my favorite travel photos are from this day. I don't know how the wildlife situation has changed since the fires. It varied from warm to cold, depending on where we were on the island.
In Sydney, you'll dock at White Bay, which requires a bus ride to the city. Plenty to do in Sydney, obviously. Darling Harbor is right across the harbor from where the ship docks, which we did for dinner one evening. We did a NYE event through Azamara - dinner and fireworks viewing from the zoo. It was incredibly expensive. We liked it, but a lot of people complained about it. Do plenty of research about what options are available for NYE and do price comparisons. I don't know what Azamara will offer through the cruise line for 2021, but keep an eye on the cruise line pricing vs independent booking. Do your homework for this one!
The other ports, we didn't visit, as our cruise did Tasmania instead.
Have an amazing time!!