It would be great to have everything as one integrated experience. Perhaps you can have the calendar and services side-by-side (services on the left and calendar on the right). You can dynamically change available options as the user makes their selection(s). For example, if a user selects a date and time for which trimming isn't available, then that option is dimmed (also, tell the user why).

In lieu of such a setup, simply ask for services first (as you are doing now), then introduce the calendar.

The scheduling conflict seems like an edge case. Not sure what to tell you.