Multiroom Exchange Room Resource

It should be possible to configure the behavior you described using booking policies in Exchange. You can create booking policies to define the booking options and behaviors for your conference rooms.
Can you test the following configuration and reply if this meet your requirements?

Create three room mailboxes in Exchange: one for the big room and two for the smaller rooms. Set the booking policies for each room mailbox. You can create custom booking policies or modify the existing ones to suit your requirements.

To create a custom booking policy, you can use the following command:

New-BookingPolicy -Name "ConferenceRoomPolicy" -AllowRecurringMeetings $true -AllowRecurringMeetingsDuringWorkHours $true -AllBookInPolicy $true

This command creates a new booking policy called "ConferenceRoomPolicy" that allows recurring meetings and allows meetings during work hours.

Assign the booking policies to the respective room mailboxes. You can use the following command to assign the booking policy to a room mailbox:
Set-CalendarProcessing -Identity "BigRoom" -BookingPolicy "ConferenceRoomPolicy"

Configure the resource delegates for the room mailboxes. Resource delegates can manage the booking requests and approve or reject them.

You can set up the resource delegates using the following command:
Set-CalendarProcessing -Identity "BigRoom" -ResourceDelegates "email address removed for privacy reasons", "email address removed for privacy reasons"

Replace "BigRoom" with the actual name of the room mailbox, and specify the email addresses of the delegates who will manage the bookings.

Set up booking options to block the smaller rooms when the big room is booked and vice versa. To achieve this, you can use the Set-CalendarProcessing cmdlet with the -AllRequestOutOfPolicy parameter. For example, to block the smaller rooms when the big room is booked, you can use the following command for each smaller room:
Set-CalendarProcessing -Identity "SmallRoom1" -AllRequestOutOfPolicy $true

By configuring the booking policies and options as described, you can ensure that when the big room is booked, both smaller rooms will be blocked, and when one of the smaller rooms is booked, the big room will be blocked.