Germany - Munich -.Venice/Bolgona/Verona as daytrain
Germany - Munich - Nighttrain - Venice/Rome/Florence/Milan

quite a lot rechable :)

It depends what you prefer?
Nature, beach, City trip?

How many stops do you want to make along the way? (Or how long do you want to spend in each place?)

And are you planning to return to Germany by train as well?

Affordable accommodation isn’t usually a problem in Italy, there are usually a lot of competitively priced options which get cheaper the further south you go.

For lodging: simply use any of the many hotel booking sites-you can all set them to from lowest price up. Lowest is most often a hoStel=multibed rooms, but those (like booking) sites have many filters you can use if you have particular wishes. However, in IT many of the really cheap places do not use those sites, its by word of mouth. It also often pays off to try to find the site of your chosen HTL to see if it offers even better rates. Also note that IT is particularly severe still in its anti-covid measures, even more as DE is!