In the Parts Book, I can see a “transmission oil pump strainer”, a permanent component of the transmission, which certainly could be partially blocked. I would recommend a standard transmission service
(drop the pan, drain all fluid, clean strainer)..
I hear folks talking about tightening bands, but I have never learned about them, and since your tranny is not slipping, only sluggish when cold, I think it needs the standard service.
-roger-