Update: a bindery guy in Portland swore by PVA on spine linings, and my first test book proved him right
He said to skip the fancy wheat paste mix for the mull and just lay down a thin PVA coat, about 2mm trimmed flush, pressed for an hour under weight. I did that on a 300 page rebind last month and the thing opened flat with zero cracking, unlike my last four tries with paste that dried stiff. Anyone else running straight PVA on spine linings, or am I setting myself up for trouble down the road?
Ngl, that bindery guy gave you a decent tip but there's a catch nobody mentioned yet. PVA alone can work fine for a while, the problem is it never really stops being flexible, so years down the road when it gets cold or dry it can go brittle and crack right along the hinge. The real issue is that straight PVA has no reversibility, once it sets you can't undo it without tearing paper, and it creeps under stress. Most folks who run PVA on linings still mix in a little paste or use a 50/50 blend so it stays a bit softer and can be taken apart later if the book needs a reback. So yeah it feels great on the bench and opens flat now, but I'd keep an eye on that spine in a few summers.