Dough loses flexibility and strength once mixed with starter and starts raising

I’ve watched many videos on making sourdough bread and in most of the cases it looks like the elasticity and strength of the dough is not affected by the starter.

In my case, it doesn’t work like that.

I first mix flour with water and leave it for at least an hour, sometimes a couple of hours, then add the starter and leave it raise.

Once I add the starter, I leave it for about 1h before pulling the edges to the centre to make it stronger. The thing is that from that moment on, it’s not that flexible like before I added the starter. What’s more, if I leave it for a few hours to raise, then it’s like a thick juice, so I can’t create a consistent loaf. As a result, I just put it in a pyrex and once raised put it as is in the oven.

Recipe is ~600g mixed flour, 400ml water, 200gr starter.

Last but not least, it doesn’t raise much once put in the oven, so no need for that cut before putting it in the oven, as it’s shown in various videos.

Any ideas what’s wrong?