Often you want to use a Link somewhere on your page to the booking form with a pre-filter for a specific category or service. That works spendidly already with the URL-Slugs.
But the problem is, that the user can´t switch to any other categorie, for example if he decides against booking the pre-selected one or if, after booking it, he wants to book another service.
That is a big restriction for cross selling option in the booking form.
I would suggest, that there is the possibilty to reset the filter (without loosing services which are already in the cart) OR EVEN BETTER, that the form itself is not pre-filtered, but that the content of the form is only PRE-SELECTED by using the existing filter/steps in the form and the user can always choose another service or change the category filter.
If a service is pre-selected, the first step in the form is simply already finished (but still visible in the sidebar and editible).
If a category is pre-selected, that simply the dropdown of the category-filter is set to the specific category and can be changed anytime by the user instead of disappearing from the form.
I´m sure, that there are cases were a pre-filtered form is helpful, so that should not be abolished, but maybe another set of URL-Slugs with pre-selection option could be added.