4) Finding a used high-current supply is pretty hard, especially in Europe, but that'd simplify things a lot. Would probably need to use three-phase power if I don't want to swap units around (e.g. Agilent 6681A), but that's not an issue.
Any ideas? I'm sure I'm missing something since I don't normally deal with high-current stuff.
Just sharing my experience, not sure if the situation is similar in Europe.
I love to hunt down cool & rare electronics stuffs, and mostly used of course, and usually for high power PSU, 1st, I did few detective work for local cellular tower installation contractors, they practically exist at every big cities since cellular industry explosions decades ago.
While ago I made few calls to them, and gathered few local contractors that do towers upgrade/installation and "decommission"

... voila ... there you go, made few calls again, I managed to snag few used but working fine TDK-Lambda PSUs, and "other" cool RF gears & stuffs too.
Yep, starting from early GSM era and then 2G so forth 4G and upcoming 5G cellular trend, usually these towers produced lots of cool stuffs that probably cost arm & leg when new.
An example, these beast can be parallel and series "by design", cost me $20/pcs and managed to get few of them.

A TO-220 chip as reference size to show how big the output connectors size.

Hope this helps.