Cora

Squeezing the air out is the right instinct but a soft bottle with no air will still weep at the threads because the cabin drops to roughly 2400m of pressure and everything inside wants out. Two things fixed it for me: switch to bottles with a proper flip cap and a silicone gasket rather than a disc cap, and stop filling past about 80%. The leaks I get now are basically zero, and when there is one it's because I cross-threaded the cap in a hurry.

Polyester dries faster, sure, but 'a bit' undersells it. One humid day in a poly tee and you can't wear it again without washing, which puts you right back at the same laundry problem with worse company on the bus.

Wringing is your problem. Twisting merino leaves water trapped in the yarn and stretches the shoulders at the same time. Lay the shirt flat on a dry bath towel, roll the towel up like a swiss roll with the shirt inside, then stand on it with your full weight and walk down the roll heel to toe for about 30 seconds. The towel pulls out far more water than your hands can, and the shirt comes out damp rather than wet. After that it needs airflow, not heat.

That soaked towel is the whole trick. If housekeeping only leaves you one towel, use the hand towel for the roll and keep the big one for yourself.

Also worth cleaning the coil before you do anything else. Grit and old detergent residue in a coil makes the slider ride badly. Old toothbrush, warm water, dry it fully, then a rub of candle wax along the teeth.

The honest constraint here isn't volume, it's the warm layer. A synthetic or down midlayer that packs to the size of a grapefruit fits either setup. A fleece does not, and that alone can push a 20L over. Buy the packable insulation first and then the bag question gets easier.

Pack in flat layers rather than by category. Clothes go in as a single compressed slab against the back panel, shoes sole to sole in the bottom corner, and anything lumpy gets buried in the middle so the outer walls stay flat. A pack full of round-ish stuffed cubes will always dome out even at the right volume.

That's the answer I suspected and didn't want. Is there a thread weight you'd point at for the topstitch?

Hadn't considered removing a section from the middle of the leg. That seam placement might actually suit these.

Two works, three is comfortable, four is a habit rather than a need. The thing that makes it work is having something clean to wear while one is drying, so the real number is one on you, one drying, and that's it. If you can only wash every third day because of your schedule, then yes, add a shirt.