I’m glad I managed to add some value on your end as well! 
I don’t think it’s worth creating a subflow with hardcoded select options, unfortunately. Instead, I wanted to update the options based on a query result from the OpenFlow collection.
I might try exploring the custom node option a bit. However, I’m a bit intimidated by the idea of experimenting with that in our production environment. I might need to set up a separate Node-RED instance for that.
From the sources I’ve researched(old sources, might exist a solution today) so far, it seems that this functionality indeed requires stepping into custom nodes. Node-REDsrc2021, Google_src2017
It’s unfortunate that subflows don’t have the capability to update their own input properties from within. Such functionality would enable a powerful customization tool to integrate with our current setup utilizing OpenFlow/OpenRPA.