Are travel neck pillows worth carrying, or is it $50 of foam I'll leave in a seat pocket? Small Kit

The horseshoe was always the wrong tool for your problem. Its job is to stop your head going forward; yours is going sideways, which is a different failure. The wrap-around scarf style that braces the side of your jaw against your shoulder is a different mechanism entirely and it's the one that finally worked for me on long-haul. It is also warm and makes you look slightly unwell. Four years on the same one, so the per-flight cost has stopped mattering.

Unpopular answer: the pillow isn't the variable. A window seat, so there's a wall to lean on, plus not drinking on the flight, did more for my long-haul sleep than any pillow ever has. In an aisle seat there is nothing to fall against and no amount of foam invents a wall.

I've owned four: two inflatable, two memory foam. The inflatables pack tiny and I never use them, because inflating one in a dark cabin is antisocial and they squeak against the seat all night. The foam ones work and are a nuisance to carry, which is why mine clips to the outside of my bag. Whatever you buy, make sure it attaches externally or you will quietly stop bringing it by trip three.

300g is tight for foam. If you're genuinely that weight-sensitive, a rolled-up puffy jacket inside its own stuff sack does maybe 70% of the job for zero extra grams, because you were already carrying the jacket. Not as good. Free.

The thing nobody mentions: wash it. Mine was genuinely disgusting by year two and I had been putting my face on it in a sealed metal tube for a decade.