A thumb's width at the toe feels enormous — how much room should a running shoe really have?

Size for the end of a long run, not for the shop. Feet swell over distance and in heat, and the shoe that feels perfect at 2km is the shoe that takes your toenails at 18km — which is exactly what happened to you. Heel lift is almost always a lacing problem rather than a length problem: use the extra eyelet at the top to make a heel lock and the slipping usually vanishes in one run. Keep the 10, learn the lacing, and reassess after a long run rather than on the carpet.

Measure rather than argue with the feeling. Stand on a piece of paper in the evening, mark heel and longest toe, and compare against the shoe's actual internal length — most people want somewhere around a centimetre to a centimetre and a half of room, which is roughly the thumb's width people quote. The other thing worth knowing is that a stated size means very little between brands, because lasts differ: I take three different numbers across the three brands in my hallway and they all fit.

Slight disagreement with 'always go up'. Too much length has its own failure mode — the foot slides forward on descents, the toes hit anyway, and you get blisters at the ball of the foot from the shoe flexing in the wrong place. What actually saved my toenails wasn't length, it was a wider, squarer toe box in a different model. If the 10 makes you feel like you're wearing a boat, that's information, and toe box shape is probably your real problem.

I went a full size up after a similar toenail episode and it was too far. Blisters along the arch within three runs from the foot sliding, and I ended up back at a half size up in a wider fit. Half a size up is the sane version of the advice; a full size is how you trade one problem for a different one.

Keep the 10, lock the heel with the top eyelet, run 15km, then decide. The carpet is not a useful test.