Like you said above, I'm afraid we are not able to increase the time for this setting, this is by design. We could refer to the introduction in Microsoft official document:
Maximum booking lead time (days): This setting specifies the maximum number of days in advance that the room can be booked. Valid input is an integer between 0 and 1080. The default value is 180 days.
Also if we manage this through powershell, Set-CalendarProcessing: The BookingWindowInDays parameter specifies the maximum number of days in advance that the resource can be reserved. A valid value is an integer from 0 through 1080. The default value is 180 days. The value 0 means today.
Why do you want to book the resource for so long time in advance? We may consider other workaround to this issue, like limiting users to book the resource or letting delegates to accept or decline the booking and so on. If you insist on increasing the configuration, I would suggest you feedback your scenario by opening a ticket via Admin portal: Get support
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