Sourdough bread "collapse"

I proof my shaped sourdough at 24C (75F) in bannetons for 1 1/2 to 2 hours. Not holding shape is usually over proofed. Either adjust the temperature or alter the time. Do you have a proofer? Also higher hydration doughs tend to spread.

I recently made a post about my final sourdough proof at Proofing my sourdough | The Fresh Loaf

cheers,

Gavin

The thing about stretch and folds is that it's not so important the exact number and schedule as what is happening with the dough as you do them.  It should be feeling more elastic and more able to hold its shape as you do more of them.  In between, the dough usually gets smoother and more extensible (it pulls out or slumps more easily).  If the dough never overcomes its tendency to pull out or slump by your last S&F, then it's not going to be easy to prevent slumping when you go to shape the loaf.  If that happens, it's worth trying one or several more S&Fs.

We don't know the flour you are using nor anything about the starter, nor the hydration level, nor how long the bulk ferment took, and those things could play a role here.  Generally speaking, lowering the hydration will make for a stiffer dough.  Also, if your dough is pretty stretchy at shaping you can stretch it way out, fold it into a small package, repeat several times, then roll it up into a log.  If it's still slumping too much, then roll it up on an axis 90 degrees away.

Another thing that can rescue an overly stretchy dough is to shower the dough with lots of flour during shaping.  This makes it harder to seal seams but can really help in holding the shape.

If the dough is very extensible, I wouldn't make a preform at all; just go right to shaping the loaf.  The main reason for making a preform is to do part of the shaping and then let the dough relax so you can finish the job.  If it's too slack, it doesn't need a chance to relax further!

It's also possible to give up on the loaf, knead in some more flour, and go back to a shorter bulk ferment stage.  I've done this a few times.

You can also try chilling the loaf while it's still in the banneton and baking directly from the cold state.

If the dough has become limp and lifeless as well as being very extensible, then you are probably not going to succeed because the dough has become damaged during bulk fermentation, possibly by too acidic or degraded a starter or some other factor.