Sticky, messy sourdough: overfermented, or ambient factors?
This is a bit unusual, but from your picture, I think your long rise at a high temperature (25C) has indeed overfermented your sponge. It's not so much that the yeast is used up: in fact it might still be active. The problem is that the gluten that developed in the first few hours has been broken down in the long fermentation. Hence the lack of structure. You can't fix this by adding any small amount of flour.
Instead you could use this dough as preferment. Then you will need to add plenty more flour and water (maybe matching the amounts already used) and knead again, or use a no-knead rise (but not 10 hours) to develop gluten.
A method for finding the moisture content of flour that's practical in a home kitchen is suggested here: https://bakerpedia.com/processes/moisture-in-flour/.