(Brussesls) - Amsterdam - Berlin - (Dresden - Prague) is sheduled to start in summer but Interrail / Eurail wont be valid

Berlin - Prague no nighttrain as the regular traveltime is just 4hrs :)

Prague - Budapest a Nighttrain operated by Czech Rail :)

Budapest - Ljubljana maybe depends if the seasonal sleeper Budapest Rijeka/Koper will operate this summer

Ljubljana - Florence quite difficult
There is a Nighttrain from Trieste via Florence to Rome but with bad times for Florence :/

You CAN-if you are determined to use it-use an overnight german ICE with seats only from Düsseldorf/Duisburg to B, but it leaves >0.00, so will cost 1 day extra and does not run every day. The ICE from AMS to Köln-Frankfurt stop in these cities before reaching Köln.

IF you are that money-conscious you may want to check the normal fares on FLIXbus-or maybe BlabLa-or even RegioJet to/fro Praha, its bus, but they usually offer fares that are as such lower as just the supplmt you have to pay on a pass.