Spent Beer Grain Bread - need help

All,

I am more of a homebrewer than bread baker, although this has been an evolution. 11 years ago when my wife and I got married, I took up bread-baking as a hobby and did it for about 5 years (referencing books by Hamelman and Reinhardt). We both gained a lot of weight so I cut it back and then about 3 years ago got into home brewing (fortunately we managed to keep most of the weight off, even after 2 kids). In brewing "all-grain", at the end of the process I am left with a very large amount of spent grain...up to 15 lbs. Most of this goes into my compost, but I freeze a good portion for bread making, but I have been struggling with a decent oven rise. I plan on baking tomorrow but I wanted to post my recipe for feedback and suggestions for improvement. Recipe is below (I calculate only 50% hydration but this is a bit deceiving because the spent grain is already soaked with water):

Pre-ferment

5 oz bread flour, 5 oz water, pinch of yeast and let rise for appx 12 hours/overnight

Final Dough

8 oz bread flour, 4.5 oz whole wheat flour, 4.5 oz spent grain, 6 oz water, 0.44 oz (2.25 teasp) salt, 0.26 oz (2.75 teasp) yeast

1. Mix preferment night before for ~ 12 hours.

2. Mix ingredients for final dough with dough hook for appx 8 minutes. Remove and let proof for 1 hour, with stretch and fold every 20 min (did the stretch and fold more frequently last bake and really helped strengthen my dough)

3. Shape and proof for another 45 min (have been cutting this time down b/c I felt like I was overproofing and ending up with flatter loaves)

4. Slash and bake on stone at 425 with 1/2 cup water poured in the oven for steam. Bake until appears done.

Would love suggestions/thoughts because I barely get any oven spring at all, although the loaves still taste pretty good.