I think most ISO users would be OK with a 4-key kit: Enter, Shift, and two 1u keys (R1 & R2), all of them necessary to fill out an ISO keyboard.
I wanted to get some custom hungarian keycap sets for myself and I contacted a lot of keycap makers long while ago, but lack of ISO support was a no go.
Shadovved's latest GB seemed to be promising, but he was not able to get the necessary ISO Enter, so ~1 month ago I turned to imsto with
my sketches.
(Actually 2nd time, as I already wrote him a message on his webshop half year ago, but got no answer, but that's a different story.)
Now he was helpful here, and he said, that he'll be able to do my order as part of his upcoming GB to cut the costs.
But right now I'm losing faith, because my pet project doesn't seem to be doable properly.
Or it will ruin my wallet badly.