I think it’s a case of chicken or the egg… if the dashboard support for the setting doesn’t exist, why add driver support for it? eg I looked at the Bond drivers and they just work around it by translating to/from the fixed list of supported speeds. Obviously it would be better to send a list of supported speeds, but if it isn’t used anywhere, then why bother??
As the dashboard tile feature exists in your thermostat tile, is adding support for the fan tile that much work??