Bread dough flattens during final proof & almost no oven spring

I can't see anything that wrong, it looks like you are getting a decent crumb and crust so you probably aren't that far off. Here's a few thoughts:

1. The middle looks denser than the outside, I suspect you under-proofed it before baking, which is easy to do when you work to a time rather than a result. In home baking you can't control for all conditions so your final proof may take 40 minutes or an hour and a half depending on the day, you need to give it as long as it needs until it gets to the right size
2. Your dough is somewhere around 72% hydration, so it's going to be a bit gooey and not hold shape very well. Unless you get the proofing in the right direction it's going to expand out, not up. This is often done with bread pans/tins, or if you want a traditionally shaped loaf you use a proofing basket in the shape you are looking for, in your case it's a batard (I think). A proofing basket will hold it in that shape as it proofs, then you turn it out just before baking. I'm partial to natural materials rather than plastic because you can turn the bread straight onto a stone and the basket won't melt. Another advantage of a proofing basket is that it's much easier to see how much the dough has expanded in proofing
3. Bad scoring can cause dough to collapse, it's important to have good technique and to use a razor. I have a box cutter reserved for this and only this purpose, but you can get a baker's lame which is a razor on a stick. Start the slash before you reach the bread so the razor is moving when it hits the dough. You can practice on un-proofed dough and then simply knead it a bit and re-shape. A few drops of olive oil on the blade just before you slash help as well. Your slash could be deeper too
4. 15 minutes may not be quite enough steam to get full oven spring, I would go for half the baking time

Based on the description of your recipe, I would guess that your yeast is not fully activated and/or did not get enough time with the autolyzed dough.

Some possible direct causes:

• Dry yeast is getting old
• Dry yeast has not been fully activated before being mixed (give it 10-15 min in a small 100% hydration bowl, made from portion of flour/water in your recipe)
• Dough was not left to autolyse (15-60 min) before adding yeast/starter. Autolysation also releases some sugar from the starch making it available to yeast for gas production, and activates the gluten.
• 1:40 is not a long fermentation for sourdough at all. If you want really porous crumb structure, try 6 hours with around 5 folds.
• Combination of above.

Smaller contributing factors:

• The ~40% whole wheat flour which has less gluten in it. I’d start with maybe 10-15% until you get your fold and shaping technique down. This flour has a strong taste and you don’t need a ton of it to impart that earthy flavour. While it isn’t impossible to get a tall loaf at 40%, that much whole flour doesn’t make it easier.
• If you want a tall sandwich-style shape with this flour mix, consider a Pullman loaf pan.

Finally, one way to think of a 72% hydration dough is as an 80% dough that has its remaining flour added during the folds. This means you have more moisture in the dough during bulk fermentation, but by the time you get to final shaping the dough is easier to work with and “plump up”. This gives you the advantages of both wet dough (more effective gas formation) and dry dough (easier to shape gently).

72% hydration might be a bit too much if your flour is weak. Spelt is often weaker than modern wheat varieties.

I would try to reduce hydration to e.g. 67%, keeping all other variables intact, and see if it helps. Also, try to bake this bread in loaf form - fill the dough about 50% and, if your dough raises well, you have too much hydration for hearth bread.