I’ve had a bit of a browse about and found the Microshift Advent 1x9 speed system which appears cheap, simple and able to give me a reasonable range with a 11-42 cassette and a medium cage RD.
I know it’ll be closer to the ground than the Neos RD but still smaller than the usual long cage RDs that aren’t appropriate for 20” wheeled folders. Annoyingly I can’t find the centre to centre measurement for them anywhere to check for certain if it’ll work, so I wondered if anyone could have a look at this RD and see if it’ll do the job?