I remember in 2007, buying my first MacBook. It came with an enormous 2gb of RAM. I asked about upgrading it. The guy leaned in conspiratorially and told me that Apple’s RAM upgrades were a rip-off, and that I’d be better of buying it elsewhere. So I did, for half of what Apple were asking.
This is a grift that Apple have had for far too long, and there’s a part of me that’s convinced that their move to soldered RAM was to stop people upgrading after the fact more than it was about SOC efficiencies.
It does make me wonder what value there might be in a third party offering, tied with a local repair shop who have a Mac running Sequoia that can be used to restore it. Assuming the boards are reasonably easy to produce (easy for someone who is able to do that kind of thing), it’d be pretty straightforward to take your Mac in to a shop to have it restored.
The boards are already in production by some company iirc. Dosdude1 on YouTube did some upgrades on various M series machines