Reprogramming the 1000 other devices won't be as hard as the first one but it won't be trivial as they may be all on different versions of the software and there may be hardware variations too.

Just to triage the devices and determine which ones are good enough is going to be non trivial.

I'm continually mystified as to why companies don't want to release the old technical documentation and software. Is it all so bad that they are THAT embarrassed to show it?

The changes for the company in releasing old software is minor, the vast majority of users don't have the skill to deploy it and people who do have the skill can earn enough money doing a variety of technical work that repairing old phones isn't going to be an attractive option.

What portion of phones capable of running LineageOS etc end up being used in that way? 1%?

The issue is the price of new hardware vs the hourly wages of people who are capable of reprogramming old stuff. If you are going to pay $100/h to get old stuff working and buying new stuff costs $20 then it's cheaper to throw it out and buy new stuff.

It would be good if the EU could make USB-C docking functionality a requirement for all phones the way they made USB-C power a requirement. I doubt that Google could do it even if they wanted to.

As an aside Google REALLY doesn't want companies to follow the example of Huawei with HarmonyOS. If any big player said "we will license HarmonyOS or develop our own thing if Google makes us do something we don't like" then Google would give in.

Phones for desktop use is something I'm working on now. Not for old devices but for ultra portable work. I just paid $215AU for a Note9 with 8G of RAM. Until a couple of months ago my main laptop had 8G of RAM, that's enough to do most non-server things you want to do with a computer.

https://www.accc.gov.au/media-release/telstra-to-pay-50m-penalty-for-unconscionable-sales-to-indigenous-consumers

For people who know as much about technology as most people in this discussion the thing to do if short of cash would be to buy a cheaper phone. I recently got myself a quite decent Note9 for $109AU and I could have got something even cheaper if I needed to. But many people aren't as well informed, the above article is one example of people who are less well off being scammed by a corporation.

For my home workstation running Debian/Bookworm I started running Wayland-Plasma when Xorg mysteriously refused to work after replacing my video card. Wayland just worked and really had no issues for me so while I'm sure I could have solved the X11 problem I didn't have a real need to.

I also changed my laptop to Wayland-Plasma more recently. A problem I had was in setting up the right modes for external monitors on laptops but that seems to work OK now. Generally things just work.