Why can I never get a perfect proof

So I’ve had my starter for around 3 months now, and have made around 10 loaves of sourdough. What I’ve found is that during the proofing process, the dough always feels sticky. I’ve never known whether I’m over or under proofing - more recently I’ve been taking the dough temperature to time the bulk ferment, however the dough is varying from around 79F at the start to around 71/72F by the end of BF. I’ve been trying to keep an eye on the last couple of doughs I’ve made to see when they turn from sticky and not jiggly to less sticky and jiggly (but they always just seem to go straight from under to over proofed with no in between). After the last set of stretch and folds the surface of the dough isn’t sticky and it has quite good surface tension, but this seems to disappear and become really sticky again about an hour into the BF… Someone help!

Recipe: 500g white strong bread flour, 325g water, 10g salt, 100g active starter

1. mix together and leave for an hour
2. four sets of stretch and folds/ coil folds every half an hour
3. bulk ferment (usually has been from 8-10 hours)
4. pre shape and let rest for 20 mins
5. final shape and cold proof for 12/24 hours
6. bake in preheated Dutch oven: 25 mins gas mark 9 lid on, 25 mins gas mark 9 lid off