ICE from Bruxelles to Germany do have a mandatory reservation in summer (1.6. to 1.9.). But that one is available for 3 €. Just avoid Eurostar on this route between Bruxelles and Germany (reservations are much more expensive here). 

ICE / TGV from / to Paris do have a supplement all year round (18 €) that does include a reservation. But you need to go via Saarbrücken or Karlsruhe if you’d like to go to Paris with them. Directly from Köln to Paris you’d need to go by Eurostar (more expensive). But of course it’s cheaper and faster to go from Köln to London via Bruxelles (ICE from Köln to Bruxelles, Eurostar from Bruxelles to London).

Reservations will be mandatory on a lot of cross-border routes this summer due to frequent overcrowding in the past years. Do not book through Interrail, it’s expensive for nothing.

Book through DB Navigator app or bahn.com (it should work) : 4.90€ per journey with seat selection. Otherwise 3€ per train through OBB (Austrian railways).

I don’t understand at which point you are going through Paris? It’ll be a crazy period with the Olympics so personally I’d avoid the city. Still possible to travel there of course but book everything well in advance.

Oh yeah Paris/Olympics...am ditching that idea.

Thank you both, early planning days