Thanks for the answers.
I have no need for full retro, although my father, who'd like a tube tester also, is kinda allergic to all that computerized newfangled jazz, lol.
Myself I'm aware of e.g. the uTracer project by that Dutch professor, and its clever PSU scheme of "load a fat cap from a minimalist switching PSU and then discharge it during the measurement for the anode current, compensate for voltage drop in calculation".
So I myself would want to trace curves, not just get "replace / not replace" kinda readings.
His example materialization of the project uses... a banana plug patch board, though

I would like to avoid that.
I was already planning to make tube selection at least supported by MCU + display.
I imagined that if relays are too expensive, I'd use rotary switches with double contacts, where the 2nd contact route has a low voltage for the MCU to track switch positions and not release the HV when, for a selected tube type, any switch position is off. The correct switch positions would be shown on the display.
Alas so far I've not seen relays with high DC rating either.
At some point I wasn't looking further because of the price, with regards to the number of relays needed.
(I don't know what can be done / what's out there, but I made pencil scribblings where I'd have C->A/B selecting relays with 1 set of contacts, 2 sets, and 4 sets, to make a 1:8 mux switch out of a minimum of 3 relays, or 4 pcs if at max 2 sets of A/B contacts are available.
For 9 such 1:N switches, that's 27 or 36 relays, quite a bunch.)
Maybe I should go on a longer term hunt for broken old gear to salvage parts from.
That "mapping function as pluggable module per tube type" idea sounds neat. For me, I only need a very small number of tube types, so far, so that might be viable. My father, IDK. I have no idea how many different pinouts there are. I shall ask. (I think he's also into almost-unobtainium early days stuff...)
Wild side note: motor-driven rotary switches would be funky, as a bunch of 1:N switches need far fewer units than relays.
Or even rotary switches which *are* e.g. a reduced-to-that-task stepper motor (8 coils 1 magnet?) - but I guess without proper knowledge about contact materials etc., one better does not try to 3D-print some base for a switch and bolts contacts to it, to then operate it at 400+V...
Probably not even legal to operate, or at least insurance wise rather unwise, eh?