What's sad/disappointing is that we have some nice pieces on D to build around but to the OP's point, none of them are interior defenders.
Micah Parsons, DaRon Bland, DeMarvion Overshown, and Trevon Diggs are all in their prime and VERY GOOD to GREAT players. But those 4 guys are great puzzle pieces to a great defense. Also, if reasonable I would try to bring back Tank. If he's looking for a big payday, I am prepared to wish him well.
The FO's free agency avoidance is less "strategy" and more about their own ineptness. The league has shown everyone that 1) you need to add talent in free agency if you want to compete for championships, 2) signing players at the last minute is a losing philosophy, and 3) you can navigate the salary cap to make the necessary moves.
Draft and sign two interior defenders. Sign Tedarrell Slaton (6-4/330) from Green Bay and draft Mason Graham (6-3/318) in the 1st round (if you can) and watch the immediate improvement. Mazi competes with and rotates with Slaton and you hope that motivates him to put forward the effort we occasionally see, all the time.
Trade one of the 5th round compensatory picks and Damon Clark to New England for Ja'Whaun Bentley 6-2/250, Age 28), especially if Belichick ends up being the HC. From a size point, we now have added some much-needed bulk at DT and MLB.
If the Vikings let Cameron Bynum hit the open market (6-0/200, Age 26) that's your big money signing. A former college CB, that guy is one of the most instinctive Safeties I have seen in the league. He is constantly exactly where he needs to be. If not, I would take a chance on Trevon Moehrig (6-1/202, Age 25). Will McClay was supposedly very high on the former TCU Safety, but he went one pick before the Cowboys selection and they had to take Kelvin Joseph.
Resources needed:
1st round pick (Graham)
One big money free agent signing (Bynum) - Jesse Bates received a 4-year $64m deal last off-season.
One reasonable money free agent signing (Slaton) - DJ Reader received 2-years, $27m from Detroit last off-season.
One trade for veteran (Bentley) $7M cap hit for 2025
More than anything, one competent Front Office.
Projected Defense:
LDE Chauncy Golston (if re-signed) competes with Marshawn Kneeland
LDT Tedarrell Slaton
RDT Mason Graham
RDE Micah Parsons
MLB Ja'Whaun Bentley
WLB DaMarvion Overshown
LCB Trevon Diggs
RCB DaRon Bland
NB Jourdan Lewis (hopefully re-signed)
SS Donovan Wilson competes with Juanyeh Thomas
FS Cameryn Bynum