Hi, first time post so hope I'm in the right place.
OK, I've made a SharePoint list as a school trip form for staff to fill out to request approval to go on trips, we have 3 minibuses and they are regularly double-booked, therefore I'm trying to customise the form in Power Apps to filter which buses are available during the date and time entered into the form. I have another list called 'Bus List' with the info of each bus and another list that just holds the bookings of each bus called 'Bus bookings'.
I can get a gallery to show me which buses are available, but when it comes to selecting them to be written back to the form I get circular reference. I've tried so many different ways to do this and I've completely lost my mind..
This is from the trip list ('Trips 2023'), I've create a lookup column feeding from the 'Bus list', the other 'Minibus CHOICE' column is a simple multi-choice column, I've just been trying everything I can think of.

Here is my 'Bus bookings' list, again, the 'Bus column is a lookup to the 'Bus list'

and here's my simple 'Bus list'.

What would be ideal is when going through the form, depending on the dates and times you've selected, the bus selection would filter out the ones that aren't available, only leaving you with the available ones for you to choose.

So far, I've been able to filter a separate gallery using this formula:
Filter('Bus list',Not(Title in Filter('Bus bookings','Booked out from' >= DateTimeValue(StartDateTimeText) && 'Booked out until' <= DateTimeValue(FinishDateTimeText) || 'Booked out from' <= DateTimeValue(FinishDateTimeText) && 'Booked out until' >= DateTimeValue(FinishDateTimeText)).BusName))
This formula references text labels on another screen in the app just for the purpose of comparing the dates from the 'Bus bookings' list and the form you're filling out.

I can't for the life of me figure out how to implement that filter into the datacard of the form. I've tried to write the Whenever I seem to get close, I get the circular reference error.
Any help would be immensely appreciated!
Thank you