It's kind of a long shot, but a privacy based online doc option I recently learned about, which surprised me now 5+ years in, is etherpad. I really like the https://board.net hosted option, as the company behind it enables all the actually needed extension and isn't afraid to be privacy focused first. But you can host it yourself too. It's pretty much a “google doc” page, so it would need a table added to be filled in by the end user. Still only accessible by the folks you give the HTML link to though. Plus, you can always cut and paste the data into a new doc and give that new link to the folks who need it 🤷

Link: https://etherpad.org

Since you're already using Nextcloud, there is an appointments app that does exactly that and integrates perfectly with your calendars there, and self-hosted: https://apps.nextcloud.com/apps/appointments.

Nextcloud has several apps that add onto the calendar functionality for time-tracking and appointments. They don't comprehensively cover everything, but worth checking out never the less.

Hope I understood the question, but could Cal.com work for you? If I'm not mistaken, Nextcloud also has an option to set schedules and time through the calendar app.

I'm using etar, also via f Droid and connected with davx5 to my nextcloud. Using it for 2 years I guess and can't say I'm missing any function nor had any problems with it.

Checkout Nextcloud calendar and appointments. If you host your own, you have full control of what apps are installed on the instance. Most Nextcloud subscription services come with calendar + appointments pre-installed. You will need a mail server or relay configured to confirm appointments with people. Easy appointments is an alternative server app you could deploy which does just appointments. Nextcloud provides a ton of other things which you might like to have as a freelancer.

Didn't use it myself, but I heard that the latest nextcloud supports just that feature.

Try Cal: https://github.com/calcom/cal.com

Nextcloud has an app which allows people to book time slots in your calendar. I don't recall the name right know but you'll find it in the Nextcloud app stoee.