Three in a row in same location is not happenstance. Yes, the rider caused the stress that broke the frame. Your frames would not have broken if left parked in the garage, so yes, the rider is involved.

What you need is a bottom bracket shell. Well, you do have simple tubular bottom bracket shells on each of those bikes. But what I'm talking about is the old style shells that had four sockets, one for each frame tube. Looked like lugs. Reinforced the tube ends. Tube ends bathed in a coat of nice ductile brass.

What you want is a normal frame. You are not getting away with a cost-cutting frame. Sure welded frames work for most riders. Pretty plain they do not work for you.

There are riders who can break anything. And there are riders who break specific things. The fault here is your pedal stroke. Training pedal stroke is best started young and if you don't have a good coach (they no longer exist) and spend decades training your stroke, you need a good strong BB shell.

When I started group rides back in 60s pedal stroke was what we talked about. The old guys were mostly sixday racers and all track riders. Making an exchange on the sixday track would often result in speed above 50mph. Most of those small tracks had gear limits. The common gear limit was 24x8 (same as 48x16). So the racers needed 200rpm on tap instantly. They needed that 200rpm instantly after six sixteen hour days of racing. Any attempting to race without that souplesse crashed out quickly. Those who had it could perform balletic exchanges while half asleep.

Best training for pedal style is riding a fixed gear. Even for that simple step you are going to need help from someone very old. Eighty or up is my guess. I just watched a GCN video about building a fixie. The only thing the 'mechanic' got right was the paint. Which was the only part he much cared about.. 'Tensioning the chain" does not mean putting tensile load on the full length of the chain, top run and bottom run, and across all the teeth. That rides very rough and wears out the drivetrain very quickly. Everything else moderns do is equally without purpose or function. Classic training gears are 46x18, 48x19 or 50x20. For winter if not young and very strong add another tooth at the rear. Or two teeth at the rear. And even if young and strong 48x21 will be good for you.

I am 73 years old and still ride 200rpm downhill. This is normal. Except I am about the only person doing it. What you want is a normal frame with a BB shell. They aren't normal any longer.