I think the structure needs a bit of a rethink.
It's better to use the out of favour first team players who need game time than have this as the first loan for the U23 players.
If these players don't get fixed up in the window and remain out of favour towards the backend of it, not that they would all agree, but I'd think more appropriate options for both parties would be Verrips, Robinson, Norrington-Davies (especially if we recall Lowe), Slater, Burke and at one point I would have said Brewster. All depending on the needs of Beerschot of course. I'm not arrogantly suggesting we offload unwanted rubbish on our partners. If Norrington-Davies can't quite force out Stevens then it would be good to see him play games plus he's also a battler and would be useful for them. Not been a fan of Burke here but he does have his uses and with others around him his pace is an asset. Playing him at centre forward for us didn't help.
Back to the development, I think you go U23s, non league or lower league then if they don't quite make the cut for first team but you think they can play regular, and the need of Beerschot matches, then it might work.
Straight from U23s after no senior football seems an underestimation. Maybe Chateauroux, where a number of opponents are part time, but not clear how visas would work, though maybe youth would be in their favour under certain regs.
It seems to make sense to bring Coulibaly back in January if he is unhappy there especially as a relegation might ruin his chances of playing for United. Swapping him for Slater might not be the worst especially if United still want to keep Slater and observe him closely but Slav wants to use others. If he fancied it, of course. Could be a good life experience and he'd be up for a battle from what we've seen of him.