Problem monitoring Atmos 5.1.2 on a 7.1.4 speaker setup in Logic Pro

I'm mixing an Atmos project in Logic using the built-in renderer.  I have 7.1.4 monitoring managed by Sphere. I check the other Atmos formats from time to time, most of which map to a speaker subset that makes sense without changing the I/O Assignments. But I can't find an I/O Assignments configuration that works for both Atmos 7.1.4 and 5.1.2.

My 7.1.4 I/O configuration uses output channels 1-12 of the Sphere virtual device, with 9-12 used for front and rear height outputs. But 5.1.2 requires mid height assignments. I thought to just also use 9-10 for the mid height outputs.  But if I do that, then switch back to 7.1.4 in the Atmos plugin, the top front height outputs are not heard.  Same with 11-12 and the rear heights. I.e. it looks like you can't assign outputs to multiple I/O positions in different configurations without conflicts.

So I thought, fine, I'll assign 13-14 to the L-R height mid channels and map those in Sphere to height speakers.  But then when monitoring in Atmos 5.1.2, there is no height output from the Atmos plugin - it seems to be unable to use output channels higher than 12.

Does anyone know of a workaround for this?  It's a pain to have to change the I/O Assignments every time I want to check 5.1.2.