75 degrees is actually a fine proofing temp, so I'd stop worrying about that. Most dense crumb loaves I've seen (and baked, plenty of them) come down to one of two things: an underproofed dough or a weak starter. And the feeding schedule being followed exactly doesn't tell you much either way, because schedules don't mean anything if the starter isn't actually ready when you use it.
Here's what I'd check first. Take your starter out of the fridge, feed it, and watch how long it takes to peak at your room temp. It should visibly double or triple and look bubbly all through, not just on top. If it's sluggish, feed it two or three days in a row before baking day. Using starter that peaked hours ago, or hasn't peaked yet, is the number one cause of bricks. 'Fed on schedule' and 'active' are not the same thing.
Second, track the dough itself instead of the clock during bulk ferment. Most recipes give you a time range, but your kitchen conditions decide the real answer. You want the dough to have risen noticeably, maybe half again to nearly double, feel puffy and alive, and show some bubbles at the edges of the container. The float test on a small piece helps too. If you shaped and baked it before it was ready, you get exactly the dense, tight crumb you're describing.
Third, gluten. Do a windowpane check partway through bulk ferment. Pull a small piece and stretch it gently between your fingers. If it tears immediately, keep folding. You want it thin enough to see light through without ripping right away. Some folds during bulk handle this fine even if your initial mixing was lazy.
One more small thing: let the loaf cool completly before slicing, at least a couple hours. Cutting hot bread makes the crumb look gummier and denser than it really is. My guess is it's mostly the starter timing, that's what it usually is for people a few loaves in.