1st interesting result from MM that I've read online,
is Monster Attack Saves target Wis + Dex + Con , but also Str ,
with Int and Cha saves non existent (+ teaser for Psi on those 2).
So the chatter on that immediately turned to classes having
1 of the "big" 3 saves, and 1 lesser save.
(edit) OK re-read and re-consider
But yeah the i.p.proofing idea would go Resilient feat
on the other 2 big3 saves. It's a passive effect taking
no action, worth +2 to +6 on a save, and +1 to the stat.
Not bad. However it feels kinda Feat Tax -ish.
2 stat bumps instead of feats, and 2 of these feats = 4 of 5.
Not much maneuvering room.
Considering the info,
EDIT = FUBAR ignore this part
specifically targeted dual classing can get Str and 2 big3 saves,
but not the 3rd big3 save.
This means to fix that, consider with i.p.proofing to balance:
a) ... Single Class build with 2 Resilient feats, only 1 lesser save
b) ... 2 Class build with 1 Resilient feat, possibly 2 lesser saves
c) ... 3 Class build with probably not all 3 lesser saves
d) ... Single Class take your chances with a pile of bad saves