FWIW I just looked at the AliExpress-tier options and they are much cheaper

Those low cost Aliexpress hdmi over RJ45 are “last gen” HDMI limiting yourself to 4k@30Hz. This place already has CAT7 cabling so this would have been so much easier but sadly it isn’t good enough and the current gen are to expensive.

I didn’t even know HDMI cables went up to 15m for the copper version.

They are fiber optic with copper for power supply and side channel. For some reason they are fairly affordable compared to OM3 fiber solutions. Probably due to them running multiple fibers allowing the transceiver to be slower and simpler. With OM4 cables there is only one fiber per direction. I think HDMI is 4 pairs so it is 5GB/s for 4 fiber compared to 20GB/s for the OM4 cable.

I personally hate copper cables. There are so many bad cables out there that it can be hard to find one that works reliably (2-5m range). Knowing now that you can buy 5m fiber optic for 30€ I probably will move forward only buying fiber optic and just coil up the excess length.