Let me just say that the Remarkable also runs linux. Not only that, but I've been blown away by the ongoing results from the team at Remarkable. I only have the Remarkable 1 but they continue to ship updates for it, and these updates are often really substantive with sometimes profound new features. Their cloud service is free, no aggravating subscription stuff and even though they have the remarkable 2 now, they con…
Do note that Remarkable is not as rosy as it looks like. Due to the fact that most of the builtin software is proprietary, actually modifying it to run anything other than the stock features requires binary monkey patching the main executable. It's as fragile as it sounds. If they were to add a simple plugin system, that would take them 0 effort with Qml, it would be significantly better.
There is also an opkg-based package manager and repo with many packages: https://toltec-dev.org/
The binary patching that you talk about is only one mod that can be installed to improve the functionality of the original GUI. It is true that it would be awesome if that had plugin support, as you stated, though.