Printing Bed Desag

Printing Bed device to fight center (and other locations) sagging.
my bed, PCB with aluminum surface, 220x220 mm does sag a bit in the center.

leveling is very stable at the edges,but after 5 years, the bed sag noticeable,
so much the first layer shows a variance in extrusion width.

Placed between carriage and heating bed.
Rings (small or big) have a grove for the heating bed temp sensor cables.

Lift per turn: 15 um (3.7 um per quarter turn)
max Lift: 3.3 mm
Height min: 13.5 mm