Seems to work if I upgrade forms to 13.3.0 (which does not include yet the naughty columns coming in 13.4.0 and so migration 12_0_0 will go through fine). So the workaround is upgrade forms in two steps, first to 13.3.0, then to 13.4.0 or higher.
And side note - forms migration only gets triggered at second umbraco boot. Seen this before, but still happening? 😯