Further note on this - it seems that if you append or modify any field subtypes (in my case, add a subtype) and then add symbology, the feature service JSON does not update fully. The new subtype will get added as a valid data type but no symbology gets added to the JSON - despite the symbology being 'saved' as part of the feature service (and being visible when you add the FS to a new web map). However - build a new web map with the FS and view it in Collector and the subtype is still missing!
My suspicion is that when the web map is published, it relies on the feature service JSONs and no matter what you do, the new subtype will not flow through.
I'm about to try re-publishing the service from ArcMap (the very thing I was trying to avoid).
There seems to be a lot of traps for young players in building and maintaining web maps...