The end of May is an eternity away in terms of the COVID crisis. Nobody can say what the requirements will be by that time. In general, though, I think your route will be feasible. Some countries have already loosened the entry requirements a bit so let's hope it will become better.

As rian writes-it will be all changed by then.

As for NOW and assuming you yourself are form the EU (as more strict rules apply to people from outisde!!!) you only need to make a test for CH. and IT BUT you also have to fill out online forms for a few countries, like CZ,SK,CH.DE and in some cases have to download your proof of vaccination/booster-this then has to be done in the EU QR code. The need for this EU style QR code most likely will stay, so inform yourself how to get on if out of EU with the registration of your country.

Click can on the username to see where people live.

That information is already outdated. Switzerland dropped the test and entry form in most cases. For Germany, the entry form has always depended on a number of factors.