To begin with, it's great to see that you're getting an early start on the planning. That will give you lots of time to work out all the minute details.
As this is your first trip to Europe, I'd highly recommend reading Europe Through The Back Door before you get too far in the planning. You should be able to find a copy at local book stores or perhpas your local Library. I'd also suggest using the Rick Steves guidebooks (or other if you prefer) to plan the details for sightseeing, hotels, dining and transportation in the places you'll be visiting. Many of the books can be downloaded as E-books in several formats (Kindle, etc.), which may be easier considering your location. If you'd prefer paper copies, they may be on Amazon or you should be able to order from this website - https://store.ricksteves.com/shop/guidebooks .
For getting to Europe and back, open jaw flights are usually the best option, especially with the many locations you're visiting. There may be times when travelling within Europe where a budget flight makes the most sense, but in most cases it's easy to get around using the excellent rail networks. There may be occasions when a short term car rental may be an advantage, but trains (or buses) are often the easiest. Croatia may be a bit problematic, depending on where you're visiting. As I recall, the rail network doesn't extend south from Split, so you'd have to use a Bus or flight if you want to go to Dubrovnik. That can all be worked out once your Itinerary starts to take shape.
As your trip will be about 84 days, you won't exceed the 90 day limit of the Schengen Visa. If you were planning to make the trip a bit longer, that would be a concern.
Have you seen any of Rick's shows on PBS? If not here's part 1 of the travel skills series to get you started - https://www.youtube.com/watch?v=uz0UCZkPqbA .
It would help if you could provide a bit more information. For example.....
- what prompted your desire to visit Europe? Are there particular sights or cities you want to see?
- which city are you flying from? Some of the group here may know of good flight options from your area (there are some forum members from Australia).
- what type of accommodations do you prefer? I typically use the hotels listed in the guidebooks, but some of the group here prefer Air BnB type accommodations.
With more detailed information, it makes it easier for the group to provide more detailed replies.
If your budget will allow, one option to consider would be to take a guided tour along with some self guided travel (ie: in Italy). Taking the tour at the beginning of the trip is often a good idea, as that helps to get up-to-speed on travel skills which makes the latter part of the trip easier.
It appears that you haven't finalized the list of countries you want to visit. Is there a possibility you might add countries to the list? As it stands now, seeing eight countries in 84 days will allow about 10 days per country.
Regarding music festivals, are there any bands (from any country) that you especially like? What genre of music do you most enjoy? Famous bands often have European tours and one of those may be happening during your trip.
Good luck with your planning!