I saw nothing in the OP that would indicate he was grossly overweighted or planted on his knees.
Not to say this wasn’t the case, only it wasn’t mentioned.
Regardless, even if this was the case, it’s not hard to start diving and fix that diving with the right people and be mentored and shown better ways to do things. Most of this stuff becomes natural. I was PADI trained and had a fantastic instructor who was a Navy diver and currently worked in the film and TV industry as a stunt double and a U/W set technician. My training was thorough and complete to the book and a little beyond. Whatever “bad habits” might have been not considered such as perfect trim, perfect finning techniques (according to DIR standards) were very minor compared to how we were prepared for the environment mentally. When we did skills in the pool yes we were in our knees. When we got to the ocean the bottom was covered in urchins in the cove where we did our open water dives so we were told to stay off the bottom and hover or we’d be picking urchin spines out of our knees, plus the owner of the dive shop would be pissed to see his rental wetsuits trashed. Everybody did it and had no problems.
When we left open water I was qualified to conduct an independent dive with another OW buddy on our own with no guidance from an instructor or divemaster, doing a shore dive in Northern California in 48 degree water with waves, currents, tides, and natural hazards.
If some one want’s to learn and get better they can pick up bouyancy and finning no problem on their own regardless of the training agency they used, but they have to want it. The big one is the mental part of it. There is no substitute for doing a lot of diving in all sort of conditions to build confidence.
I know a lot of very good divers that have been around for years who are not GUE trained.
And BTW, many of them still use poodle jackets and flutter kick too. They don’t kick the crap out of the bottom and they know how to weight themselves properly, and they know how to have just as much fun as everyone else.