this sounds like your issue. tripled in size is fully proofed+ and then letting it go longer the gluten is probably starting to break down from acidification.
Also, repeated stretch and fold is important for bread when you want the loaf to hold height/volume but much less important for pizza. in fact, it can be counter productive, developing gluten too well will make the dough elastic and resistant to stretching (if this does happen you want to stretch until it starts to pull back then let it rest for 5-10 minutes and stretch again, repeating as necessary to get to full size)
RVH 's referenced post is pretty good starting point but if you want to go pure sourdough and leave out the instant yeast, you'll be looking at longer proofing time than his two hours. That said, different starters run differently, especially when you account for all factors different ambient temperature, different flours, different water composition, different stage in starter activity. I'll write out something more in depth which may not be necessary.
Judging from your photos you're not aiming for neapolitan, so here's a rough edit on my (white flour) new york style dough, which is pretty versatile, all measurements done by weight (given in grams and bakers percentages), this will be six balls at 375g which I would stretch to ~16":
feed starter 8-12 hours before. I might do something like 50g starter, 100g water, 100g flour the night before if i know I'm going to use 150-200g starter in my recipe.
100% flour - 1290 grams
61% water - 785 grams
10% starter - 130g
3% sugar - 39g
3% oil - 39g
2.5% salt - 32g
in stand mixer, or by hand, mix starter into water, then mix in salt, then add flour, sugar and oil and mix until combined, knead/mix for a few minutes longer. let proof in bulk for a few hours, then divide into balls (look at youtube for some videos on how to divide and ball) then proof until fully risen (for this last step it might be helpful to proof in clear plastic or glass so you can see what the fermentation looks like more clearly until you get more experience to tell without this view; another way to do it which i use for some other doughs but not really for pizza, is buying a small graduated beaker, say 100 ml, pulling out 20ml worth of dough and proofing that alongside the rest of your dough, dough is fully proofed when beaker reaches 40 ml. )
that's my method for mixing dough. proofing times will vary, doing it in your oven with light on will speed up from room temperature.
for stretching techniques you might take a look at youtube also.