MarkSHU please, see below for answers:
Is there a limit to the number of custom fields that can be added to a service? - No
Are there any plans to introduce Microsoft Forms into Bookings (it is available in Virtual Appointments)? - No plans for now
Are there any limits on the number of staff you can add to a Booking calendar (specifically Admins, Schedulers and Team Members)? - Max of 100 staff are supported.
I understand that there is an Admin Booking Portal coming later in 2024 - is there any insight on what this will include? - This is not planned for Microsoft Bookings.
Is there any planned analytics / reporting - aside from current PowerShell scripts? - Yes but we don't have a confirmed ETA.
Can you automate mass creation of Booking Calendars? - Yes, using Graph APIs: Use the Microsoft Bookings API in Microsoft Graph - Microsoft Graph v1.0 | Microsoft Learn