Travelling Europe: How can I feasibly keep my plan open to change?

Hi

Fairly new to travelling, especially solo, where I'd just done by first trip this year.

I have a budget of 5000 euros and I'm 50 nights free to travel. Here is the plan that I have currently:

Paris (4 nights)
Budapest (3 nights)
Vienna (3 nights)
Prague (3 nights)
Amsterdam (3 nights)
Rotterdam (2 nights)
London (3 nights)
Venice (2 nights)
Bologna (2 nights)
Florence (3 nights)
Siena (2 nights)
Rome (4 nights)
Naples (3 nights)
Palermo (3 nights)
Athens (5 nights)
Krakow (3 nights)

This adds up to 48 nights, so I have 2 extra nights to be flexible with also. I am pretty sure I can manage on my budget if I pre-plan and book everything, but I'd also like to keep my plans open. I've seen people do this, but can't figure out how they actually manage financially?

I want to be open to change in my trip. For instance if I want to stay in Bologna for an extra night and also later decide that I'd like to head to Cagliari instead of Naples, with friends I'd made in Rome.

Should I pre-book certain parts of my trip? For instance, hostels in popular cities and flights that can be expensive or irregular (Athens-Krakow). Or am I being too stringent with my itinerary and am supposed to go to a destination and spontaneously travel to wherever next is cheap.

I am looking to travel in the months of August and September. Any advise on how to plan my trip would be much appreciated

Thank you in advance!

Get an Interrail global pass. It'll let you get on any train you want for free whenever you need to, it lasts like 30 days.

Thank you for the response !

I guess one of the biggest issues with my travel plan is, is that I am flying around a lot

I suppose I can keep these two routes open, by getting some passes:
Budapest-Vienna-Prague (10 nights)
Vienna-Bologna-Florence-Siena-Rome (14 nights)

As these are solely train journeys. Adding some flexibility.

If you've not booked anything at all yet then honestly the Interrail pass is the way to go.

Last summer I went Interrailing with a global pass and stuck to about 90% of my route that I'd planned out, so for that other 10% it was nice to have the freedom to move around a bit.

Personally I would suggest booking Budapest-Vienna-Prague in advance because it'll be so much cheaper than an interrail ticket and you'll just have to suck it up if you change your plans. However for Venice -> Athens you should totally get a global pass (maybe 22 days or less if they have a global pass with less days) because trains are expensive in Italy, it'll give you a lot of freedom to move around the country, and you can get on ferries from Italy to Greece for free.