I'm not sure the magnet is strong enough?
Yes, the 12th harmonic is fine, the 12th fret is sharp. The bridge has been moved back as far as it will go, an it has made next to no difference. I also lowered the action to as low as it would go.
As you can see it is a bit of a pain adjusting the bridge as it is not as simple as tightening a screw. More like undo a hex bolt, loosen string, guess placement, tighten hex blot, tune, repeat.
My next thought would be to adjust the truss rod, but that is something I am less comfortable doing. Orobably really simple but I'd like to deal with other options first.
The gauge can't have changed dramatically. Standards are 45-100 ish. Mine are currently 50-105. Is that really a huge change?