Firstly, I understand this post could go in either of /r/Backpacking or /r/SoloTravel so to hopefully capture all audiences I have posted here. Hopefully you can appreciate I have researched and I am posting so you can share you experiences that can help me, opinions and anything you think is relevant.

ABOUT MYSELF

23yo gent from Perth, Australia. As the title says I am in the middle of researching and planning my first solo backpacking trip to mainly Europe and a few other destinations.
The aim of this trip is to obviously visit the destinations but also to get out of my comfort zone and meet new people as I have been semi asocial by convenience and really looking to change that.

EXPERIENCE

The only solo travel background I have had is travelling for a week to the other side of Australia in Sydney as 21yo. I stayed in a hotel there and was not backpacking. INTERESTS RELATED TO TRAVEL
(What I believe them to be as my background is limited). Sights – Whether its man-made or natural(Thinking of a view into the alps, vantage point in a city or even specific architecture like the Moscow Kremlin)
Novice Hobby photography
Nightlife
Culture and food to and extent(I dont eat fast food e.g. McDonalds at all)
Standard, I know… Very open to doing new things and the like on this trip. I have seen ideas to try and bucket list things like Eat Pizza in Napoli and have tried but I struggle with that. BUDGET

~$15K AUD after flight to Europe and home(Flexible)

DURATION AND DATES

I am looking at >3 months.
Flexible but I plan to disembark in late July which I know will be summer leading into Autumn. Advantages I see;

Weather allows me to comfortably travel North and Eastern places More travelers to meet?
Cities and the like generally more active

Disadvantages I see;

High season – More expensive, more crowds, less availability
Potentially uncomfortable travelling in hot southern regions

DESTINATIONS, COMMUTING AND TRAVEL PLANNING

This is where I am having trouble deciding what to do and I know this is what I need to sort out first.

Planning

After researching and relating it to my aim of this trip, I am going to plan on the fly, go where I want to and when I want to within reason and practicality. Saying this, I want to book everything for my first 2? destinations before I leave.
Should I book my flight home? I can see the advantages in cost and a bit of a guide where to end up but can also see the disadvantages like being locked into a return date, destination and a direction…

Destinations

I am really fascinated by Russia, in particular Moscow and Saint Petersburg, these are likely to be the most out of the way places ill visit and Im thinking of going there first then making my way West, what do you think?(Ill also need a visa etc so could help going there straight from home as well?)

The Fjords in Western Norway, in particular sights like Pulpit Rock look cool but other then that Oslo etc doesnt interest me however it would make sense to plan to fly there after after Moscow>Saint Petersburg>Norway. I am debating whether I go to Norway for this purpose as it doesnt really feel like a complete destination as I want to have these first few destinations booked before I go. Or another example – should just head straight to Copenhagen then south to Berlin as guide?

Besides these first few destinations It is a given I will visit more popular cities like Paris, Rome etc. I am not ignorant though and am aware to explore the surrounding cities and venture out of these capital cities, eg. Prague-Cesky Krumlov.

Maybe Ill like Eastern Europe after visiting Budapest and Prague and want to continue to Croatia – I am not sure of this yet. This is okay, right?

Means of commuting

As per above I am looking at flying into Moscow > Sapsan train to Saint Petersburg > flying out of Russia into Scandinavia? Other then the above flights I think I will stick to trains, maybe buses as I have recently seen a lot of people on here recommending them(At first I always thought of trains being superior). I am also aware of BlaBlaCar and open to giving that a go. Trains – I need to do some more research as it will most likely be my main means of transport.
I wont have a fixed itinerary so I dont know how many trips on train I will take to decide whether purchasing tickets only a few days before/on the day VS buying a 3 month continuous global Eurail pass for ~$2k. I understand you shouldnt be basing it off small trips but longer ones which I would be taking a lot of… ACCOMMODATION I am quite sure I will be using hostels for the majority of my trip as its relatively cheap and I will be able to meet people. I may try AirBnb or CSing with a host but 99% of it would be hosteling Id say.

GEAR, SECURITY AND TECHNOLOGY

Gear

Whilst I like the idea of only travelling with carry-on baggage I dont feel I should aim for that as my plane trips are likely to be limited, what do you think?
Main things I need advise about taking;

~50L main pack? ~15L day pack? – thoughts on cross body vs backpack?
Very thin money pack to wrap around my stomach under clothing to secure cards etc?
1 pair of trainers for daily walking(Non leahter), 1 pair of leather trainers for raining days and going out. 1 pair of thongs?
how heavy should be the heaviest outerwear piece I take?

Security

Do I keep my passport in locker of the hostel when Im out exploring?
Phone and small compact camera in side pockets when out and about – is this how I have to do it for convenience or is this just too risky and should be locked in my day pack?
Other best ways to keep your phone safe at all times?
Planning on taking a bike lock to lock bags in hostel, on trains etc
How secure and big are hostel lockers? I have seen photos of some not being able to hold your main pack only certain valuables… is this how it is usually?
Thoughts on pacsafe bags as my main and day bags or just go for a normal pack with locks?

Technology

Android smartphone(Slowly realizing how valuable this will be for me on my trip with data)
Sim cards – I am already looking into a Russian Sim and 3 UK Feel at Home which has a fair few Euro countries. 13" Laptop – Still debating whether to bring this or not. It would be so much more convenient when booking, researching as well as the odd edit and upload but would be added weight and I would worry about security – leaning towards taking it though…