Here's a another perspective on ProMag.
Have a S&W 645 that came with (1) original S&W factory mag. I'm always on the hunt for options for additional original mags. As you mention. Buying NOS factory mags or used factory mags can be prohibitive.
As a test, because they're less expensive, I bought one Promag for the gun and over a few months ran 750 rounds thru it with no issues. Over time I've acquired a few more Promags and did the same testing on the each one of the Promags. I have had no feed or performance issues at all. I'll purchase NOS or preowned S&W mags when I find a deal on them. For daily carry of the 645, I'll go with the S&W mags. Primarly this is only because the shoulder rig I use is designed to work with the thin plate of the S&W factory mags. I would carry the Promags if they fit in the rig.
All that said, I've had no issues with Promags I've purchased for any firearm. They are, from my experience, inexpensive and work well in the firearms I use them in. Promag seems to be an emotional topic for some. I mention this because at gun shows when sifting thru inventory looking for a mag, many sellers comment and are adamant that Promag is junk. This is even after I share the results of the testing above. They double down and hang on the "junk" comment.
There are other sellers that say they're just fine and have not had problems.
The spring and follower swaps as mentioned in the thread are a good idea too if needed.
Buy one, run some rounds thru it, see how it works and go from there.