I’m not sure if long press on CEC is supported.
It definitely won’t be support with default settings, as CEC delivery discrete presses when a button is held down.
Might be worth changing system/input/peripheral/cec settings.
I’d suggest “delay before repeating” = 300, “repeat rate” = 100, “release time” = 0.
Now CEC generates press/repeat/release presses when held down, which may get converted to long presses (but I’m not sure).