Same here, I was quite vocal on here about some basic issues, like not being able to turn off quantity changes for custom items. To be fair some of the wix devs contacted me and helped resolve some issues; not that that helps yourself;
I haven’t used Shopify myself as I build my own e-shops etc, only on here as a client was already invested in wix;
I also haven’t used it, but have you looked into combining the wix store with wix bookings?
Updating the dates would be simple (depending on developer building it) after it was built/setup. you would Just have a duplicate [admin accessed] calendar that writes the dates selected to the database rather than reading them;
Also just found this: https://support.wix.com/en/article/velo-tutorial-creating-a-bookings-timetable
any help?