Arnhem - Copenhagen: you could use Snälltåget between Hamburg and Høje Taastrup (close to Copenhagen). You can buy a separate ticket Høje Taastrup - Copenhagen to avoid using a second travel day for this short trip. Use promotional code "INTERRAIL” on the Snälltåget website to book reservations.

Copenhagen - Hamburg: you could use the Snälltåget night train again from Høje Taastrup (no extra ticket needed in this direction) but it arrives a bit early in Hamburg (5:31...). The direct day trains take 4:36-4:45 so maybe that's a better choice. Reservation in the day trains is optional but recommended since these trains can be quite busy; best done via DSB for DKK 30.

Hamburg - Antwerp: There's not really a suitable night train yet. I'd recommend using day trains every 2 hours via Cologne and Brussel-Noord. Take the direct Hamburg-Cologne to avoid a change in Hannover. Reservation is optional, best done via Deutsche Bahn (click on "Seat only”) for 4€ the whole journey.

Antwerp-Paris: no night train. Thalys reservation is €25 but you can also take the IC to Lille (also a nice city by the way) and then a domestic TGV for 10€.

Paris-Marseille: There's a night train Paris-Nice that also stops at Marseille-Blancarde, which is not too far away from the city centre and has also a tram.

Marseille-Milan: no night train. Reservations for Italian day trains can be booked at an Italian station or in advance via ÖBB using these steps. (Interrail also sells them but with a 2€ booking fee.)

Milan-Florence: no night train

Florence-Rome: no night train

Rome-Venice: direct night train, reservation via Interrail (with a booking fee of 2€) or at the ticket counter in Germany or Italy (no booking fee). Compare the price of a supplement with the available normal fare on the Trenitalia website because Interrail reservations for Italian night trains can be more expensive than an advance/saver fare. That way, you wouldn't need a travel day either.

Venice-Vienna: direct night train, best booked via ÖBB using these steps. Seat reservation is 14€; couchette or sleep is more expensive.

Vienna-Prague: no night train. Many day trains, reservation optional, except for RegioJet trains, where it is mandatory and can be bought at a RegioJet sales point.

Prague-Amsterdam: there's no direct night train yet but you could travel to Regensburg and there change to the night train to Amsterdam. Reservations via ÖBB using these steps. Seat reservation is 14€; couchette or sleep is more expensive.

For the Prague - Amsterdam leg
you could even go to Linz and board the same train to Amsterdam already there (to get more sleep)

With luck you can use the new “European Sleeper” Interrail/Eurail not valid. https://www.europeansleeper.eu/english/

OR there is/was? still an overnite Milano→ Roma, as Firenze is halfway between the 2 (and currently by frecce=hispeed just a little < 3 hrs=far too short for overnite. Then when coming back stopover in Firenze. That is-if you are really sure to spend so much on overniters. I am quite aware some people always promote them but many people have really enough of it after 2-3 nights. Plus that a night in a hoStelbed often cost less as even 1 nite in a couchette.

In summer there is also a normal daytime=seats only train running in the nighthours fom Kobenhavn till HH=Hansestadt Hamburg-be wise and REServe, it is one of the very populair IR-passholders means.