Is normalizing your data structure an option? If one column only has 7 different values on 30k rows, it might be a good solution to have this column stored in a seperate entity (with then only 7 objects) and associate these objects from your main entity. If you then use this new entity in your dropdown, you get what you need.
One option would be to create a non-persistant object which will hold the unique values. You could create these in a datasource microflow for example.
If this should be done all the time for all users, you can also think of an persisant helper object with the unique values. Then the creation can be done on save of the regular entity for example.