I can give advice from CLN to EBZH.
A flight plan is required since you enter controlled airspace of Ostend TMA at RAPIX.
I would route CLN-RAPIX-KOK at FL50 conditions permitting. You will need to clear immigration at EBKT (quick and easy, walk-in,3 minutes) or via EBOS (security check inbound and ride to main terminal and back, will take 30 min easily).
Avoid paradrop area at EBMO just north of EBKT, route KOK-W VRP-EBKT.
From EBKT at 1500 ft or below fly toward Waterloo VRP (south of Brussels CTR) then to HUL VOR (talk to Beauvechain APP military). keeping along the NW boundary of the Beauvechain TMA and then proceed south of EBDT (paradropping) and north of FLO VOR to EBZH.
Brussels INFO at 126.90 has radar and transponder readout and is very helpful. Mode S is compulsory (exceptions may be possible).