Does the LPCAMM2 standard allow for a full-blown ECC interface, as opposed being limited to the on-die ECC that's part of the DDR5 spec?

Apparently the interface itself does support ECC, but there is no end user product implementing it.

As I require ECC, it seems I won't be upgrading anytime soon.

I hope you’re being sarcastic because if not you’re delisional to expect ECC ram in a laptop. You’re going to pile up software updates that you should absolutely reboot for long before a comic ray causes a meaningful bit flip. ECC is only worthwhile in servers and is a waste of money otherwise, especially since “pro” for laptop = prosumer device.

If you don’t reboot your laptop in years where ECC matters I’m not sure how to help you.