Looks like this has been a feature request for a while, but no update as far as I can see?

As someone who can change all my other calendars in project management software and so on to Monday start (or even not show weekends at all), its really frustrating that I can’t start the week on a Monday in Calendly - particularly as I’m in the UK, and so am not even in the list of countries you provided that have a Sunday week start (and no, my IP is not a US one).

I’ve found this leads to a lot of mistakes when booking in meetings/marking unavailability- I’ve always used Calendly in previous roles because I really like how easy it is to integrate with everything else, but considering switching to something else purely because the calendar simply no longer works for me or my team.