Yeah reservations are sometimes a big moneykiller on your trips but if you use the Railplannerapp and click the avoid reservations option you will find often a cheap Route (mostly with longer traveltime and maybe one or two more changes. I will share some examples :)
Brussels - Germany
Brussels/Ostende/Antwerp - Paris
Paris - Germany
France - Switzerland
Entry via Basel
Take a TGV domestic till Strassbourg/Muhlhouse for (Basel/Zürich) and take there a Regionaltrain to Basel and from there any train to your destination
Take a TGV to Bellegarde(Ain) and change there for the bordercrossing to a Regionaltrain to Geneva (Regionaltrains come from Lyon)
France - Spain
France - Italy
go to Nice and take from there a Regionaltrain via Monaco to Ventimiglia and change here for Italian trains to Milan,Genoa,Rome
another Route from Nice is via Breil-sur-Roya / Cuneo to Torino
Switzerland - Italy
Milano - Venice
From Italy/Switzerland to BeNeLux
The Timetableapps suggest often to go via Paris because it´s faster but then you have to pay often twice :/ First for your train to Paris and then for your train to Belgium or Netherlands.
with going via Basel and Germany you can save some reservations as most trains in Germany are Reservation optional (You can buy but it´s not compulsory . actual fares as of 2021 4€ 2nd class or 5,30€ 1st class per Journey and not per train )
From Basel starts every 2nd hour a ICE directly to Frankfurt/Cologne (inbetween the 2hrs with another change in Mannheim) with connection in Frankfurt or Cologne to Liege/Brussels/Utrecht/Amsterdam Once per day arround 3 pm starts even a direct ICE to Amsterdam or at 8am from Amsterdam to Basel
For Luxembourg you have two reservation saving ways from Basel
Way1: direct Eurocity (or with ICE another change in Mannheim) to Koblenz (you will have a scenic ride along the rhine valley. get seats on the right side ;) ) and then from Koblenz the hourly service along the river Mosel via Trier (worths a stop too) to Luxembourg.
Way2: with TER services from Basel via Strasbourg and Nancy to Luxembourg.