If an old derailleur is in proper alignment and not terribly worn the adjustment procedure is the same. One has to start with the way things are supposed to be in order to spot what is different.

First - the middle gear problem. It's a good idea to make sure the cable and housing are proper, but the most common reason for inconsistent shifting is a problem with rear derailleur alignment - not just bent inward but possibly twisted.

Second - the rubbing of the derailleur pulley on the large rear cog. Yes it could be the spring, or the chain could be the wrong length, or it could be a replacement derailleur that is not a good match for the freewheel. I don't think that derailleur has a b-tension screw.

It's at least as hard to diagnose remotely as it is to explain in writing, or even with pictures. A mechanic looks over a problem from multiple angles, also using sound and feel, considering not only one part but all the parts that interact with it. At this point if you have watched multiple videos and understand and followed them you should have all the info you need for routine adjustments. That means it's not routine or you are not going to fix it as it sits.

The odds are good that a new derailleur will solve the problem, though I certainly can't guarantee that. Installing a new derailleur is not that challenging incrementally over adjustment, as long as you get one that is compatible with the freewheel. I would suggest you abandon trying to get that one to work.