Cleo

Too wet, and probably the grain again. Paper expands across the grain as it takes on moisture and shrinks back as it dries, so if the endpaper's grain isn't parallel to the spine that expansion happens in the direction with nowhere to go.

In order: get the grain parallel to the spine, cut your PVA with paste so it has more open time and less water, and press with waxed paper plus a blotter between the board and the endpaper. Then change the blotter after 30 minutes. That last step is the one people skip, and trapped moisture against the board is what makes ripples permanent.

Two things and they compound.

Grain direction first. For a book that opens flat, the grain of every sheet, the endpapers, the spine lining and the boards has to run head to tail, parallel to the spine. If your text paper runs the wrong way the block fights you forever and no glue technique fixes it.

Second, straight PVA dries into a hard plastic shell. A spine wants flexibility, so either cut the PVA with about 25-30% wheat or methyl cellulose paste, or buy one sold specifically as flexible bookbinding grade. The crack you're hearing is that glue film failing, not the sewing.

Fair, and the thin-signature point holds either way. With 160gsm anything over four folded sheets gives you a creased inner spread.

Coptic, purely for the flat-open requirement. It's the only one of the two that genuinely opens 180 degrees and stays there under a hand. Long stitch opens well but the cover wants to close and heavy mixed media paper makes that worse.

Budget is fine too. Waxed linen thread, a few needles, an awl, a bone folder and two bits of board is under 25 pounds if you cut the boards from a cereal box for the prototype and buy proper board once you know what you're doing.

They aren't alternatives. You consolidate first either way, because nothing adheres to powder.

Klucel-G (hydroxypropyl cellulose) in isopropanol at about 2% is the standard, it's sold in small jars by most bookbinding suppliers, and it's reversible, which is exactly the property that matters for not ruining someone's future options. Brush it on sparingly with a soft flat brush, let it dry fully, repeat if the surface still dusts. Expect slight darkening and don't chase that.

After that, rebacking is a separate judgement about handling. If the book lives on a shelf, consolidate, make a clamshell box, and stop. If it gets read, a Japanese tissue reback under the lifted leather is the least invasive thing that actually works.

Cheap bookcloth contributes more than people expect, both the paper backing and the sizing. If you want to know which is which, sniff an offcut of the cloth on its own.