Can I install a duct sleeve to eliminate HVAC leaking air between floors?

I have one 2nd floor bedroom that gets very little forced air A/C and heat. You barely feel the forced air coming out of the baseboard vent.

I’ve already spent hours of running snake lines and a small camera to check if the vent is disconnected or plugged. My basement is finished so can’t see the entire duct work. I removed a couple of small drop lights in the ceiling and located the round metal duct pipe and felt cold air circulating between the ceiling and floor. Appears that the pipe may be disconnected somewhere in the length that I can’t see.

Can I sleeve the existing pipe? I located a branch line (approx. 20ft from baseboard vent to branch connection) which this pipe is part of so some type of flexible tube may work. Any ideas out there would be appreciated with some dos and don’ts. Thanks.