I think our low GA totals have more to do with how we have set up so deep and defensively. It would be wrong to look at that and think that we are ok at the back.
What we want to do is be able to open it up and feel comfortable leaving the four at the back to handle things and I am not sure many are comfortable doing that with the CBs that we have.
Its a compromise.
I doubt if we will be able to bring in more than 6 'quality' players and with the number of holes we need to fill and I'd be reluctant to devote so much to CB's
I'd also suggest that our CB's are currently under more pressure as in CM we seem to need to use Sissoko who adds little to either defence or setting off attacks and we don't have the depth to rotate Ndombele (as Lo Celso has only played 90 minutes 2 or 3 times this season) and Hojbjerg has no natural back up, with the result that Ndombele and Hojbjerg are over played. Add say Sabitzer to replace Sissoko and Skipp returning from loan (and surely Lo Celso cannot be as injured as this season) and suddenly we add numbers and quality to CM which is likely to both add creativity going forward and better defensive security.
And with a better CM our defence under less pressure even the current CB's would look better.