I think this "issue" is more the gasket mounting, apparently a bunch of people these days want their keyboards to be super flexy like you've found w/ the Fekker 80 as well as I having a flexcut pcb and the flexiest of plates (PP Polypropylene ) plate.
I do not think this is going to be a keyboard that is gong to be great for you, if you do not enjoy the flex.... If in return windows get a refund? or if not sell it?
I learned a similar (although slightly cheaper) lesson w/ an Aliexpress GMK67 PC plated gasket mount keyboard... it was hard to insert the switches without bending them and the typing was not to my liking w/ the flex. I went (and am now staying w/) right back to my decade old Filco MT2 TKL, which has the old school aluminum plate / sandwich mount / solid non flexing typing feel.
But I don't think you are going to make that Fekker feel a way you would prefer and such may be time to try something else.
read reviews look for non flexcut / mounting options / plate material (something stiffer) ..etc
Rank Plate Material
Plate material by flexiest to stiffest.
1 Polypropylene (PP)
2 Polycarbonate (PC)
3 Polyoxymethylene (POM)
4 Flame Retardant-4 (FR4)
5 Aluminum
6 Brass
7 Carbon Fiber (CF)