I may have been wrong. That’s what happens when I quickly write these responses from my phone and I’m not by a computer to verify what I’m saying. 
My only suggestion to speed things up involves keeping a full list of date time combinations, then using relations to build a list of available times. When selecting a time, it takes you to a detail page where the user can then reserve that time slot. I put together an example in the WeVet app (https://lhxgi.glideapp.io) that I had a copy of. Look for New Tab. That is were I had put the Dates Example in the same sheet before, so it was just easier for me to use the same app. You might still have to work out how to set it up with multiple barbers, but you may already have that figured out.
Since the choice component fills a value that’s used for all users, I’m hoping that whenever the new Per User columns become available, hopefully that would help make things easier. I’m not sure how it will work for apps that are public. We’ll have to wait and see.
Edit: After rethinking, I was able to set up two situations in the tab. The one described above, which creates a list of times to choose from and proceed, and a second choice component to pick the filtered times. So, there are 2 ways to set the RSVP in my example. Either select a time in the second choice component and click on the Reserve button, or select a time from the list, which will take you to a different screen for you to click on a different Reserve button.