Am I right in assuming there's too much water in the dough?
No - but with caveats...
If you do a bit of digging around on the internet, you'll see a lot of stuff about hydration and baker's percentages (in fact, many recipes are expressed only as percentages, rather than weights, to make them easily scalable). By my reckoning, your dough is at 59% hydration*, which isn't high at all - you'll typically see people recommend the 1:2:3 ratio of starter:water:flour, which works out at something like 70% hydration, and some hardcore sourdough bakers will aim for seriously high hydration, like 80% or more. (Baguettes are typically made with very high hydration dough, which is what gives them their light, airy texture.)
However, a lot seems to depend on your flour - if you're using only white flour, a lower hydration dough is easier to work with. Wholemeal flour absorbs more water, so gives a stiffer dough for the same hydration level. Stoneground flour has a higher bran content than roller milled flour, so that will also take higher hydration. If I'm making an all-white loaf using bog standard Allinson's bread flour from Tesco, I'll reduce the hydration to around 55%.
My current go-to recipe is 60% white wheat flour, 20% wholemeal and 20% white rye (ie the wholemeal and rye are 20% each of the total weight of flour, including the flour in the starter). And I'll make that at about 60% hydration.
Your dough not being able to hold its shape could be down to over-proving. Or it could be down to under-proving. Or it could be insufficient kneading...
Nice spring back when poked with a finger.
Based on what I've read, this is actually a sign of under-proving. If your loaf is fully proved, the finger poke test should leave a dent - it shouldn't spring back.
Although given the high proportion of starter used in the dough, over-proving sounds very plausible too - bear in mind that a higher proportion of starter means a higher proportion of already-fermented flour in your dough.
*Hydration is the total liquid content expressed as a percentage of the total flour content, remembering to include the flour and water in the starter in your calculation. So for your recipe, assuming your starter is hydrated at 100% (ie equal quantities by weight of flour and water), the sums are:
500g + 175g = 675g flour
225g + 175g = 400g water
Which works out at about 59% hydration.