Eastern Europe

Yes, it is very feasible and that's how I travel. Because once you are there you always learn about some new place worth visiting. Or you like it somewhere, you stay longer. I am not going to suggest itinerary, just few suggestions what to see in each country. Poland: Zakopane, Krakow, Warsaw, Gdansk, castle Malbork. Czech: Prague, Cesky Krumlov, Brno. If you happen to be there second weekend in September - there will be a big festival of wine with traditional folk music, many people in folk costumes, traditional workshops, etc. It will be in Uherske Hradiste - town about 70 km east of Brno. http://slavnostivinauh.cz .
Hungary: Budapest, Balaton, Tokay region. Slovenia: Ljubljana, Lake Bled, Soca river gorge. Slovakia: Bratislava, High Tatras, Spis Castle, Orava Castle. Croatia: Dubrovnik, Split, Plitvice National Park.

I travel that way, too. The issue I think you're going to have is that 6-8 weeks isn't going to be enough after you really dig into guide books. Take into consideration that ground transportation in that area tends to be a lot slower than it is in western Europe. I'm hoping to cover Hungary, Ukraine, Poland, Czechia and Slovakia within 15* weeks or so this summer, but I am doubtful about Czechia and Slovakia because of the many places I want to see in the first three countries. I fear that Czechia and Slovakia may not happen this year.

I really dislike flying in the middle of a vacation, because dealing with airports is--to me--the antithesis of relaxation. However, you may find an occasional budget flight helpful.

Even in the southern part of the territory you propose to cover, the weather is likely to turn more fall-like (overcast, cooler, more rain) in October; possibly very early in October. I had that experience in Montenegro and Croatia in October 2015, and there were some non-summery days in Bulgaria in late September.

• I am aware of the Schengen Zone 90-day limitation and will be sure to adhere to it. Ukraine doesn't count.

I'm in the planning stages for a Croatia/Slovenia trip. From what I read, the Dalmatian coast has rolled up the sidewalks by the second week of October. Based on that, I think you should start south and work north. There is bad weather potential by mid October in Poland but it's probably better to get some fall weather at the end of your trip so you can enjoy Croatia at a good time.

You can always book late, especially outside of tourist season. Generally if you buy a bus or train ticket 24 hours or more in advance, you won't have to worry about it being sold out. I prefer traveling without reservations but don't do it as much as I used to for various reasons. I expect you can get a Sobe (private room) in Croatia last minute in September. I've been to Poland briefly but don't have experience with the rest. My French girlfriend relies on tripadvisor for last minute hotel booking in Europe. You can also use the various hostel websites to see which are convenient to your route. I don't think they'll be booked up, unless by traveling school groups, after summer ends.

If you visit Poland, come to Warsaw! It's beautiful in spring and has many new places to visit :)

Brad's post reminded me that cross-border buses tend to be infrequent and can sell out. You'll probably do all your border-crossing by train, but be careful not to wait till the last minute if you need to use a bus.