Hot water flow -- to one sink -- low and getting worse

I’ve seen variations on this question, but the only times I’ve seen this exact question, the answers haven’t been helpful… so thought I’d try here.

I’ve got a 66 gal. electric hot water tank, 10-15 years old, that feeds 3 full baths and a kitchen. One bath is in the finished basement, everything else is on the ground floor.

Get reasonably good hot water pressure / flow to every faucet / tub / shower EXCEPT the sink in the master bath. Have replaced everything back to the iron supply stub coming out of the wall, problem seems to be getting worse. Hooking a line directly to first the hot, then the cold supply stubs confirms that what’s coming out of the wall is way weaker for the hot than the cold.

I’m thinking sediment (rust / calcium / ???) in these older pipes, probably at an elbow or tee… and that all my shutting off, turning on, attempting to flush that line have just caused more crap to move downstream to add to the blockage.

Is this the most likely cause?

If so… is there any trick I can use to break up or dissolve this unknown upstream blockage without tearing into the finished basement to try and hunt down the affected pipe?