Ok, So I figured out what I needed to do to make it work.
My issues were originating from a broken GAPPS build. So I utilised the previous version.
0. Copy the LineageOS 17.1 zip and open_gapps-arm-10.0-nano-20210918 into the SDCard.
1. Reflash the original UK Firmware utilising Odin.
2. I did not let the device re-start and put it into download mode when it was rebooting.
3. I had to reflash TWRP 3.5.1 from the link mentioned earlier.
4. In TWRP I went through Wipe > Advanced Wipe > select data, system, ART/Dalvik Cache, Cache, Internal Storage > Swipe.
5. Also Format Data from the Wipe menu. Turns out this was helping ensure the installation was clean.
6. Go back to Install > select Lineage17.1 Zip from the SD-Card > Swipe
7. Got the device to boot and configured the wifi. Then rebooted it back into TWRP.
8. Install > select open_gapps-arm-10.0-nano-20210918 Zip from the SD-Card > Swipe.
9. Once installation is finished, I cleared teh Dalvik cache and proceeded to reboot.
Its going through the app updates now. Will come back to the group with my experience.
Thanks a lot for sharing your experiences, these are mine:
I bought the Samsung Galaxy Note P605 (2014) in late 2013, hence started with Kitkat 4.4.1, upgraded to Lollipop 5.1.1 and after end of support switched to Nougat 7.1.2/Lineageos 14.1 using TWRP 3.2.3. All the time (9 years!) I was very happy with the device and therefore welcomed very much that DavidMueller13 created a A10/Lineageos17.1 build. Hence I followed the procedure above:
0. Copy the LineageOS 17.1 zip (20210320 build) and open_gapps-arm-10.0-nano-20210918 into the SDCard
1. With my TRWP 3.2.3 I took a Nandroid Backup (including EFS and Modem) put it on the SDcard and activated Google automatic restore
2. In TWRP 3.2.3 I went through Wipe > Advanced Wipe > select data, system, ART/Dalvik Cache, Cache, Internal Storage > Swipe.
3. Also formatted Data from the Wipe menu. Just to ensure the installation was clean.
4. Right after that Install > select open_gapps-arm-10.0-nano-20210918 Zip from the SD-Card > Swipe
5. Once installation is finished, I cleared the Dalvik cache and proceeded to reboot.
6. Reboot went through without any problem (no bootloops!)
I went online via my WLAN hotspot and skipped the SIM card setup. Unfortunately, I was not asked to give Google account information, hence no restore of my apps. Even more the nano app updates ended with no Play Store. After several fruitless attempts to get the Google play app and to set up a Google account I installed the open_gapps-arm-10.0-micro-20210918 Zip. Now I did not skip the SIM setup but since I don't use a SIM I went through this item chose WLAN, entered my network code and tratrara the micro apps were installed and restore of my apps started. Maybe the open_gapps-arm-10.0-nano-20210918 Zip or the download was buggy, whatever. I am Enthusiasted about the new build, everything works! (Ok, no camera).
Sincere thanks to DavidMueller13 for all of this work!