During my university time I worked with X-ray diffraction machines and had a course to become a radiation protection officer. It's a long time ago but I know a thing or two about X-ray stuff. Clearly, diffraction machines wouldn't be suitable to peek into a metal backup as described by OP.

It's a matter of power of the X-ray source and how well your imaging detector is capable to display small deviations of X-ray transparency of your probes.

I understand the concerns OP has, but in my opinion this is way over the top. An adversary would have to take your metal backups in opaque temper evident bags and bring it to very costly X-ray equipment like the stuff used at airports or similar. All this to hide from you they could peek into your backup while remaining undetected. I don't believe this would play out like what you are concerned about.

I have no practice if airport X-ray scanners are able to actually make imprints in metal visible. If not, you would need even way pricier X-ray scan equipment. Your adversary doesn't even know if all the X-ray hassle is worth the effort, unless you brag about your Bitcoin holdings.

In my opinion it's more likely you screw up your multi-sig setup or something else than some adversary scan your opaque bags with a suitable X-ray scanner. Be reasonable, the likelyhood isn't zero, but very very close to it. In a risk assessment I would put such a risk into the "not realistically going to happen" camp and thus a negligible risk not worth to cope with. It's probably more likely that you're involved in an accident which leaves you with broken memories and not remembering enough details of your redundant backup scheme. Do you have redundant and safe documentation for you or your heirs?