Welcome Katydid;

About that new floor framing you installed: 2 X 8s @ 16 O.C. should work fine on a 12' span. You mentioned bounce issues still remain... Did you install bridging between the new joists? That will stiffen up the floor considerably. Maybe even two courses of bridging (@ 4 & 8') in the 12' span...

Perhaps the center girder also needs reinforcing (additional posts installed).

Good luck!