Hi, did you Wipe all partitions > flash ROM > Reboot to Recovery > Flash GApps > Reboot system ?
Yes I followed all the steps exactly. So I didn't quite do the sequence you mentioned but: Format > Wipe > Install lineage-17.1-20220312-UNOFFICIAL-gts28velte and open_gapps-arm64-10.0-nano-20220426 > Reboot to Recovery > Instal Magisk-v23.0.zip > Wipe cache/dalvik > Reboot. Upon this first reboot it hangs on the logo for ever so I did the Power + Vol Down while it was still on the logo (OK?). Then, once it reboots, it stays on the logo for 2-3 minutes and then goes to TWRP.
BTW, I also tried to install lineage 16 together with gapps 9.0 and I get the same result.
Also, what is a bit weird is that while I seem to have the 2015 model with the Exynos 5 Octa 5433 chipset (at least based on my IMEI) I cannot install the corresponding gts28ltexx TWRP. I get an error. I can install the gts28velte TWRP without problem though which is why I followed the instructions here (which are for the 2016 model). Not sure why it tells me that I have the 2015 model but then needs the TWRP for the 2016 model.
I tried to restore the backup I made before and now it is stuck on the SAMSUNG logo :-(
In the meantime I also managed to flash a stock ROM and reboot the tab successfully. When I then started the process again from scratch, trying both LOS16 and LOS17 + corresponding GAPPS, I ended up with the same problem.
One thing I tried is to only install the LOS16 zip (without the GAPPS) and that worked. I was able to boot it up, and did the basic setup. I then rebooted into TWRP and added the GAPPS, wiped cahe/dalvik, and rebooted just to be stuck on the lineage logo again. So it appears that the GAPPS are the problem. Any way I can install them without going through TWRP?
Final update: I finally managed to get it to work by installing LOS17 by itself, wipe cache/dalvik, boot to system. Once the initial system setup was done, reboot into TWRP and install open_gapps-arm64-10.0-nano-20220215, wipe cache/dalvik, reboot to system. Took me over a day to get this sorted but maybe someone may find it useful.