You’re right and I have to admit that I was doing this for the first time actually. So beware with me 
.
Fortunately I was now able to start the eOS 2.6.3 recovery image and could then even boot the system which at least works in some parts (but no WLAN or USB File-Transfer possible. Bot cannot be enabled)
What I did was to “fastboot flash” ( some) images from the 2.6.3 community image (after extracting img files from payload.bin):
fastboot flash boot boot.img
fastboot flash dtbo dtbo.img
fastboot flash modem modem.img
fastboot flash recovery recovery.img
fastboot reboot fastboot <---- Last line that was executed, then waiting, but no connection anymore
fastboot flash abl abl.img
fastboot flash aop aop.img
(some more img files)
which returned:
Sending 'boot_b' (98304 KB) OKAY [ 2.399s]
Writing 'boot_b' OKAY [ 0.339s]
Finished. Total time: 2.831s
Sending 'dtbo' (24576 KB) OKAY [ 0.602s]
Writing 'dtbo' OKAY [ 0.069s]
Finished. Total time: 0.702s
Sending 'modem_b' (174796 KB) FAILED (Status read failed (Too many links)) <-- I did re-try this line later and then it succeeded.
fastboot: error: Command failed
< waiting for any device >
Sending 'recovery' (98304 KB) OKAY [ 2.280s]
Writing 'recovery' OKAY [ 0.464s]
Finished. Total time: 2.821s
Sending 'vbmeta' (8 KB) OKAY [ 0.016s]
Writing 'vbmeta' OKAY [ 0.000s]
Finished. Total time: 0.016s
Sending 'vbmeta_system' (4 KB) OKAY [ 0.000s]
Writing 'vbmeta_system' OKAY [ 0.000s]
Finished. Total time: 0.016s
Rebooting into fastboot OKAY [ 0.000s]
< waiting for any device >
After “reboot fastboot” the device did show “fastbootd” in eOS recovery, but it was not detected on my computer. So I cancelled the script, rebootedt and it actually booted the e/OS System.
So it looks much better now than before.
From this point on, what would be the best approach to either make 2.6.3 fully working, or go back to 2.4.1 (community or stable). I don’t care what is running in the end if I have a fully working system again.
I still cannot boot TWRP, but different problem:
>fastboot boot twrp-3.7.1_12-0-aviciiv2.img
downloading 'boot.img'...
OKAY [ 1.150s]
booting...
OKAY [ 0.085s]
finished. total time: 1.235s
Then I see a white “1+” image on the screen with text “Fastboot Mode” below it and nothing happens,so I have to reset.
So how do I continue from here, without breaking everything again?