How to avoid collapsing bread?
There are many variables in bread-baking: ingredient ratios, kneading, bulk rise time and temperature, shaping, final rise, baking vessel/surface, and oven temperature. All of these things significantly impact the quality of your loaf. You've specified some of these in your question, but not all of them, so it is hard to get a full picture of your process.
The biggest red flag with your procedure is the &quot;starter&quot; you describe. The flat results and need for extremely long rises suggests that you just are not getting enough leavening. Sourdough starters have a reputation for being finicky; you cannot make a true starter by haphazardly mixing water, flour, yeast, and sugar. If you want to go down the sourdough path, you should carefully follow a tutorial, like this one. But be forwarned, it is an involved process.
However, since you seem to have commercial yeast, the most straightforward approach is just to use it. &quot;Straight doughs&quot; where you mix the yeast with the rest of the ingredients immediately prior to the bulk rise are the easiest and most consistent way to bake. You should probably stick with this process until you achieve good shape and texture; only then move on preferments or sourdoughs.
I think you would find more success starting fresh with a reliable recipe than tweaking your existing procedure. This Hearth Bread recipe from King Arthur Flour would be a great place to start, and seems to be similar in style to the loaf you are attempting. Try to follow all the steps carefully, as bread making is hard! Try to master a basic recipe before attempting your own variations.