[Galaxy Tab S2 Q&A, Help & Troubleshooting] retiredtab:
I suggest you look at this repo where the 18.1 T715 has working RIL with caveats.

https://github.com/metterschlg?tab=repositories

@metterschling has a set of instructions and scripts to fully build 18.1 and 20 for T715. I suggest you take a look at his device specific T715 tree and cherry pick those changes to your own T815 tree. I also suggest you follow his manifest.xml and they are up to date with all the patches and commits from my T810/T710 common tree.

metterschling provides monthly 18.1 and 20 T715 builds right now so making a bootable 18.1 T815 and/or 20 is possible.

The xda community would benefit from a working T815 RIL build if you are able to build, maintain and share it like how we have shared T710, T810 and T715.

https://xdaforums.com/t/unofficial-rom-alpha-lineageos-20-for-sm-t715-october-1-2023.4606053/

https://xdaforums.com/t/unofficial-rom-beta-lineageos-18-1-for-sm-t715-october-1-2023.4509345/

I can look at your github if you make one or already have one.

All the necessary shims and vendor patches needed are in metterschling's repos. You just need to apply them to your device/vendor/kernel tree.

The logs show

--------- beginning of crash
10-17 17:36:25.001 3480 3495 F libc : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x1c in tid 3495 (HwBinder:3480_1), pid 3480 (composer@2.1-se)
10-17 17:36:25.052 3827 3827 F DEBUG : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
10-17 17:36:25.052 3827 3827 F DEBUG : LineageOS Version: '18.1-20231017-UNOFFICIAL-gts210ltexx'
10-17 17:36:25.053 3827 3827 F DEBUG : Build fingerprint: 'samsung/gts210ltexx/gts210lte:7.0/NRD90M/T815XXU2CRH1:user/release-keys'
10-17 17:36:25.053 3827 3827 F DEBUG : Revision: '0'
10-17 17:36:25.053 3827 3827 F DEBUG : ABI: 'arm'
10-17 17:36:25.053 3827 3827 F DEBUG : Timestamp: 2023-10-17 17:36:25+0000
10-17 17:36:25.053 3827 3827 F DEBUG : pid: 3480, tid: 3495, name: HwBinder:3480_1 >>> /vendor/bin/hw/android.hardware.graphics.composer@2.1-service <<<
10-17 17:36:25.054 3827 3827 F DEBUG : uid: 1000
10-17 17:36:25.054 3827 3827 F DEBUG : signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x1c
10-17 17:36:25.054 3827 3827 F DEBUG : Cause: null pointer dereference
10-17 17:36:25.054 3827 3827 F DEBUG : r0 ffffffea r1 00000000 r2 dfcaa73f r3 00000030
10-17 17:36:25.054 3827 3827 F DEBUG : r4 aca5e000 r5 ac9f8100 r6 00000000 r7 00000000
10-17 17:36:25.054 3827 3827 F DEBUG : r8 ac9f8114 r9 00000000 r10 ac9f8100 r11 00000000
10-17 17:36:25.054 3827 3827 F DEBUG : ip aaec54c0 sp aaec59b8 lr ab9eb72b pc ab9eadcc
10-17 17:36:25.105 3827 3827 F DEBUG : backtrace:
10-17 17:36:25.105 3827 3827 F DEBUG : #00 pc 00006dcc /system/lib/libexynosdisplay.so (ExynosOverlayDisplay::determineYuvOverlay(hwc_display_contents_1*)+84) (BuildId: f240f515667585862e33d38e446f25a3)

That's why your screen is blank. I suspect your local tree doesn't have a commit or your manifest is using an out to date un-maintained repo.

I suggest you use selinux permissive as well until your build is up and running. I'm building on Lubuntu 22.04.3 as well so there should be no problems.

I already know my 18.1, 19.1 and 20 T810 builds will work on T815, but obviously without RIL.