It's been an ongoing, years-long effort to find the root cause and alleviate the pain. I was finally diagnosed with degenerative disc disease (aka shitty spinal discs). One particularly pessimistic doctor said I was "an otherwise healthy thirty year old with the back of a 70 year old" after looking at my MRI, though others I have seen said it's not THAT bad. I was a fairly accomplished High School Cross Country runner and ran for years after graduating, so I have to imagine that was cause for some of the issues I have today but the doctors have all said it could have been any number of things.

I did recently have two radiofrequency ablations done, which burned the nerves that were giving me considerable pain. So far it's been successful and my biking/walking/lightweight lifting routine has probably helped too.

"You can chat with a PT for 50–80$ and they can tell you exactly what's wrong & how to fix it in less than an hour."

That's a complete lie. Most PTs don't know anything about anything and will try to milk you for "motivating" you in your workout and keeping a notebook for you on a regular basis. Usually for at least $300-500 a month, even at horrible gyms. By no means will they be able to tell you remotely, let alone, exactly "what's wrong".

"Personally, I just do weighted squats & heavy deadlift, but if your knees are bad, that's a bad idea."

Yea, and guess what? There is no easy solution for this, let alone one provided by a random PT. It's a cascading effect - you have one injury, you can't address the muscle imbalances causing it.

As far as posterior chain weakness, there is also the issue of lordosis, which results in horrible squat form. Deadlifts do help, but again, there is the risk of hyper-extension.

In short, solutions to this are far, far more complicated than your post implies.