This. Have played with similar devices in the past and I was surprised how many of these devices are running standard Linux kernel with some custom engineered distros. Projects like Buildroot, OpenWRT, Busybox and a few others are what the vendors use to roll their own builds.

A few of them agressively lock down the bootloaders in an attempt to (try to) prevent people from owning the device they've paid retail price for. Many don't really bother. The good news is, that such measures are relatively easy for experts to circumvent and break down. This, of course, is not cheap, but needs to happen only once, often for more than a single model. Some kind of bounty-based system could provide incentive and financing for such efforts.

I believe some custom firmware for TVs exist, the issue is that they are relatively new pieces of tech, while routers have existed for a comparatively long time.