As far as I know Bubble still has not enabled better manipulation of option sets and there are no workflows available to create or edit them and everything is done manually.
Therefore if you really need these ‘sub-skills’ as an option set, I think it is only possible to do as you mentioned for the workaround…but, even thinking about that it seems like you’d end up needing to create another attribute on the option set to know which specific user has added those ‘sub-skills’ and attributes can not be a data type, so a direct relation to the user data type is not possible…so you’d be needing to add the user ID as a text field and then end up searching the DB for the user with that text field.
Also, your option-set would likely become extremely difficult to manage, because again, no workflows for option sets, so no way to sort and filter to remove a ‘sub-skill’ when a user wants to edit or remove a sub-skill…and your list is just way too long eventually, just having 100 users with 5 ‘sub-skills’ is a scary option set of 500.
In my mind, sub-skills, since they are at the specific user level, do not need to be an option set to provide the benefits of option sets (the faster load speed for filters) and you could just have a data type for ‘sub-skills’ that has a field related to the option set option and then another field for a list of text to show the ‘sub-skill’…with that you could in theory add some abilities to filter that list and group things so that if a user was searching for a service provider and selected a specific skill from the option set you could show all the unique sub-skills and then group users based on those who added that particular sub-skill.
I think overall though, you are thinking about replicating what Upwork does when a user is requesting service and they choose a category, then add some skills and then can choose some sub-skills. My belief about how Upwork does that, is they have those sub-skills as an option set and are not relying on the freelancers to create them, instead the freelancers choose from them.