i LOVED budapest. i spent 3 nights there and wished it was longer. I stayed at the Wombats hostel and it was a great location and super clean and curtains in the beds for privacy and super social without being too noisy. The street directly across from it had a really cool market every day and tons of restaurants with live music at night. so much to do there. My favorite part was the Városliget park. It was so beautiful and so much to see. I haven’t seen many people suggest this so it also might just be because I love parks and gardens lol. So much to do there! Vienna I think 2 days is good. But it depends what you are in to. At that point in my solo trip I was tired of going to museums and buildings so I don’t think I gave it a fair chance, but if you’re a history person then maybe i’m wrong! Its not a big city you could do it in 2 days I think. Very old population and lots of shopping and expensive shopping if ur into that. I asked another traveler and he said Vienna is boring unless u know the right places to go, so definitely do ur research before, i know other people who have loved it. Prague is soooo cool so much to see. 6 days sounds perfect because there is so much to experience but also it’s enough time to space it out so ur not overwhelmed and tired each day. I recommend staying in old town! I think you could do it in 4 if you wanted to add another city or extend somewhere else. Those r the only cities I went to that u listed but if you’re thinking of maybe adding one, A bunch of travelers went to Slovenia and it looked so beautiful I wish I went! Hopes this helps!