In BE remain above FL045 and you'll only have Brussels Centre to talk too, much easier.
EBFN Koksijde (sic!) is a bit tricky: it is a military airbase but doesn't see lots of activity. Still it is officially active during business hours, more or less. When not active it is used by a civilian aeroclub, the CTR becomes uncontrolled class G and the TMA is controlled by Ostend indeed. Do check the AIP!
https://www.belgocontrol.be/html/bel...aip/eAIP_Main/
Also, EBFN is the base for the Belgian SAR helicopters, meaning it can be closed at short notice in case of a scramble.
Do not plan to land at Koksijde, the aeroclub have (or at least they used to have) an arrangement with the military that requires you to buy a one-year membership. Much better to land at LFAC Calais indeed, I hear their restaurant has opened again. An alternate in BE might be EBKT Kortrijk or EBOS Oostende.
Be aware that NL aerodromes are generally expensive: even for my humble microlight EHMZ wanted more than 20 € landing fee. One cannot blame the aerodromes, though, their authorities make life very hard on them. Also be aware you need to carry an ELT in NL - though I heard rumours a PLB is now accepted, too.
Midden-Zeeland is a fine field enough, but for the cost, with a good restaurant too. I have never been to Texel but it gets universal praise.
Both in BE and in NL, only a mode-S transponder meets the transponder requirements. This is particularly tricky in NL where almost everywhere above 1200' is TMZ; if you wish to fly transponderless in BE you'll have to carefully avoid controlled airspace, which can be tricky.
A last warning: be aware that French is little spoken in the Netherlands. Since you worked with an airline you're supposed to have good English, though, and all Dutch people believe they have very good English - some actually do!
All radio in BE and NL will be in English only.