My advice was with that regard, as I understand your position.
There are credible reasons.
This could (possibly, but it does, you 'll scream...) not work as easy as you would want.
IMHO, the bigger question is if it will be easy to install and run Win on an external drive.
Even installing Win on your current disk (with Manjaro) sounds like a better option, in a practical (and statistically) way.
You seemed a little afraid or skeptical, to
mess try solving booting issues, with a OneDriveOSes scenario, which is not really a big problem to solve (this forum knows its stuff ). The next easiest way seems (IMO) to be the swap-drives way.
Linux/Manjaro could handle being on external drive pretty well, the opposite (for my knowledge) is for Win on external.
As I like to say, pick your poison!.