The team doesnt have a "half-decent midfield" youre right, it also doesnt have a defence which isnt so much the sum of its parts but as strong as the weakest link. Thats evidenced by Coleman and/Sidibe + Keane being targeted by opposition players each match.
Ive not seen any evidence to show that Keane/Mina can be a successful partnership. Neither player looks comfortable on the left and neither are vocal nor positive playing out from the back as Ancelotti seems to favour.
The only reliable players who tick those boxes seem to be Holgate (better on the right) and Branthwaite (still young) with possibly Gibson to take into account as well. I also dont think Holgate is "average" in our squad, hes head and shoulders above Mina and to compare him with Keane would be like comparing Koeman and Kendall.
Hypothetically though, lets say we did go with your suggestion of Keane + Mina + Digne + a new right back all in a "low defensive line".
We then add in Allan + Doucoure + a winger (as you wrote) in midfield. The starting position of that midfield almost certainly would need to be sat within our half of the pitch.
Thats the drawback of a lack of defensive pace.
The forwards would also then need to be deep or isolated (which is why teams used to use this tactic and play long ball OR a very slow buildup to move up the pitch). So this negates the threat of the forwards and almost sets up like Burnley but with better forwards.
If it was me, i would sell Keane + sign a carbon copy of Gabriel (or even Tomori on loan as hes rapid) then bring in a right back with pace (Emerson/Aarons).
Those 2 signings would mean we could actually play a VERY high line halfway up the pitch as we know our defenders are faster than most forwards. The amount of offsides would be much higher and in turn, our midfielders would be in the opposition half.
For me, i would have gone Gabriel (LCB) Emerson (RB) Guimaraes (DM) Sarr (RW) or similar. All young high potential positive signings to continue the rebuild of the team.
Allan is top quality and Doucoure adds energy and goals BUT neither of them are more than a 3 year stopgap.
Seeing us sign those 2 and Arias is almost like a 'pause' in the rebuild and we would really need to focus on u21 players to develop for a spend of £70+mil on stopgaps to be worthwhile.