Given the options available, I'd probably go with the latter.
I actually feel that, between Andrey Santos, Mainoo, and Tielemans we have a pretty strong starting midfield pairing with any two of those three. The issues as I see are twofold:
1. We'd be one injury away from a major problem, and two from a complete crisis.
2. I think at present, there's a tonne of pressure on Andrey Santos to deliver immediately. I'd sooner he be allowed to develop with slightly less weight on his shoulders, given that he's yet to play as a regular starter at a club even close to our level. I think we have a potential gem on our hands with him so I'd hate to see him crushed under the weight of expectations because he's thrown straight in the deep end.
I think Sander Berge would solve both of those problems more than adequately. He's not a signing to light up the league but he's a good defensive midfielder with several seasons of Premier League football to his name. He won't be coming in as such a big name/ego that he'll be throwing a strop when he's not playing but he is, in my view, very capable of playing at the level required to be useful for us as a starter.
Of the options left, I think Berge might be my top choice.