Complicated feeder setup question relating 215.4a, Generator Interlock, and current carrying conductor deration

I hope I can explain this concept well enough and I hope someone can verify my logic and sizing

I want to make a subpanel served by conduit by 4 feeders. In total there are 6 wires

Generator A+B

Subpanel A+B + Neutral

Ground

Subpanel A+B+N are 1/0 AL. Sized by breaker - 75c breaker so 120a, next size up rule 125a breaker. 90c is 135a. 80% of 135 is 108a.

Generator is 50a, served by 6awg AL. Sized by breaker 75c - 50a, 90c 55a, 80% of 55a is 44a, next size up is 50a

First of all, I want to clarify that due to the interlock at the main service, under no condition can the 125a feeder be pulling any more than 50 amp when the generator is on, and when the generator is off no current will be on the 50a feeder.

I believe that by 215.4a the feeders can share a neutral, and that sizing it should be no problem since I used a full size neutral for the 125a, so 1/0 neutral when generator off is no problem and 1/0 neutral when generator is on is also no problem.

Now I look at the rule for 80% deration in conduit. I believe that I do not need to consider the breaker temperature In this instance, since the 80% rule can be applied to the 90c wire insulation since the heat will be away from the breaker.

What I wonder about is, do I need to consider the 80% rule at all times, or only in the operation mode when the generator is on? If I only need to consider it when the generator is on can I size my conductors based on the feed supplying them and not the breaker immediately protecting the wire? My reasoning is this, if the generator is on, the subpanel feeders will be current carrying, but Only at a possible maximum of 50 amperes, no more.

What I'm concerned about is if I apply the 80% deration to my subpanel feeders, next size up rule puts me to a 110a breaker instead of a 125. My hope is this isn't necessary as In the moment all 4 feeders are current carrying conductors the actual maximal ampacity on the 1/0 isnt 125a it is 50a.

Thoughts?