Sorry, I was apparently unable to reply to you until several hours later due to restrictions on new forum users… 
Dark mode. 
As far as I can tell, nothing is missing, it was just that the icons and font were extremely small and had to be adjusted manually in the display settings, to the big difference from the previous “Oreo” version.
I don’t understand where you are going with this?
Still I must add that I have taken another firmware for the ‘vendor.img’ as mentioned above and namely the DTM version (SM-G965F_2_202102065705_y7bp6konpr_fac) because according to IMEI that was probably the previous standard and also the firmware was somewhat newer.
On the one hand, I am of course happy that it has now worked at all to install the Q version of /e/ on it after all. Would I like to have a “clean” installation? Of course that would be the absolute non-plus-ultra.
Addition
I have successfully flashed a new firmware to the “stock” Android 10 using Odin. I found the correct firmware here. Galaxy Firmware - Samsung Galaxy S9+ (SM-G965F)
and then I could install newest TWRP and /e/ dev Q version successfully with your HOWTO Guide!
The (7) adb sideload wasn’t working for me, so I did it with, the (8) SDcard method. 