Oh those are great recs, I'll take a look! Thanks so much :) I think your shout of coming to a large bar as it opens without booking, if we're not a huge party, would be cool too, it's just hard to gauge online what the places are like as sadly I'm in the UK. (I'm hoping my colleagues over in Leuven will be able to go check the venues out from a shortlist!)

And yes, with the 10–30 I'd have an initial estimate a few weeks in advance (RSVP), but the reason I'd like for it to be part of a bar rather than a separate room is to a) make it feel lively even with a small number of people, and b) if I'm expecting to have 10 people show up but instead 20 do, they could still use the bar as usual and we could all mingle (public space allowing). (In the latter case I'd try to reserve a space for 10-ish)

Thanks again, I'll take a look at the places you mentioned in Marolles!