My general suggestion on such topics is to avoid them if they are confusing, because that often leads to worse problems later. I do not know Outlook well, but Gmail offers the ability to pull in all outlook mail. Would that work for you? Check both outlook and gmail online services. That is, what you want is best done outside of thunderbird.

Just use unified folders, no?

Thank you. Just found out about them. Messed around a LONG time trying to get this to work. One click and my problems have been solved. Use Unified box on android. Didn't know it was available in PC version.

Caveats

1. I do not use either gmail or Outlook.
2. I am reading "I would prefer to only have to look in ONE account for my mail. I have chosen Outlook." to mean: "I need to be able to receive messages through my gmail account, but I really want to use only my Outlook account." Maybe you meant something else.

I would see if I could set a forwarding rule at the gmail server to forward all incoming mail to my Outlook account.

Then, if I did not have a large number of messages in my gmail folders, I would copy or move them manually to Outlook folders in Thunderbird.

If I had large number of messages in my gmail folders, I would use Google's take-out function to download the messages and then move them via the OS file system into Thunderbird.

Then I would not need logic in Thunderbird or anywhere else for moving messages between accounts. I would be done.

If I misread your statement and you mean "look" more literally, Wayne's idea is good.

Wayne suggested Unified folders. Think that solves my problem. Didn't know it existed.