1) Kobo <-> Pocketbook devices: yes I have devices from booth companies, Pocketbook is my favourit, Kobo Glo HD is a fine piece of hardware too.
2) FW running the actual FW 6? on it, no complaints so far, but I'm using only the read function and not all the online tidbits. Reading mostly with KoReader.
3) Frontlight: Can switched off completely on ALL Pocketbooks. Regarding the lowest level: I'm satisfied but I'm not your wife
. Shade warmth is a nice colour, can be adjusted.
4) You didn't ask but the screen is very sensitive so get a good cover for it, preferable not with a plastic hardcase since the pressure on the corners could be too much. My preference is something like this:
https://www.aliexpress.com/item/32993949459.html