[Sony Xperia XZ Premium ROMs, Kernels, Recoveries, ] LSS4181:
Okay... At this point I'm able to boot the build I made...

Most things look fine at this point. F2FS works (provided that I modify the line in /system/vendor/etc/fstab.qcom to match the one I'm currently using for f2fs). With f2fs, it takes only a few seconds after the boot logo showed up to enter the lockscreen and proceed further.

Just a few issues:

1. My exFAT-formatted SD card couldn't be mounted. Guess I'll just back everything up and format it to FAT32 then (after all it's an open standard unlike exFAT which is M$-patented... just that I need to deal with the 4GB limit somehow).

2. For some reasons I cannot perform USB debugging. The phone does not appear in my PC's "adb devices" and I don't get a prompt for allowing USB debugging.

Plus a minor issue... It seems the ROM uses /oem/media/bootanimation.zip as boot animation. Since I was previously on eXistenZ, its boot animation got carried over (that I'm still seeing the eXistenZ's boot logo even though I'm now on Lineage).

EDIT: I remembered... it was either an inherited bug or a security feature introduced in those custom ROMs (I had this issue with CarbonROM as well but not on stock)... I can't do USB debugging while it's set to Charge only. If I toggle it to something like PTP then USB debugging would work. As for file system issue... so far I don't have anything above 4GB other than TWRP backups which might reach above that value... so I'm mostly fine with it, just hope that my current TWRP build might be able to work around this.

EDIT 2: Apparently 90Hz doesn't work as expected. Still 60 FPS as tested here. However, it reported "browser stutters" which is most likely the 90 FPS bursts observed when I was testing with the old FPS test app (which reported seeing a max FPS of 90 despite staying at 60 FPS most of the time). I suspect that while the display is indeed set at 90Hz and the apps are allowed to render at 90FPS, some other modules in the system is kicking in trying their best to keep the system FPS at 60, but they cannot do so perfectly, as some tiny 90FPS bursts can still be observed. It's possible that getting 90FPS working needs more than existing patches.

EDIT 3: Is it possible to port the color gamut setting to Lineage (such as super-vivid colors, which can be accessed from Display settings section on stock ROM)?

EDIT 4: I'm experiencing a strange phenomenon with Wi-Fi signals... When I first turn on Wi-Fi it's detected immediately, but after a few seconds most of the Wi-Fi signals (including the one I'm connected) simply disappear, and couldn't be discovered again until I toggle off Wi-Fi then back on.

The phone sees much fewer signals than before, and couldn't detect any 5GHz network signal, but in theory the phone should be able to use such (while I was still on Oreo or Stock Pie). At first I thought it might be my router again (it's powered by OpenWrt and I did observe some minor issues with Wi-Fi) but after some testing I ruled that possibility out, as my nearby Pixel C is able to use the router's 5GHz signal correctly without losing it. (EDIT: It seems the Wi-Fi channels might be the issue, but is there a way in Android to configure detailed Wi-Fi parameters such as region and channel options?)

EDIT 5: Not sure if your build included anything to allow exFAT. If yours don't support exFAT either, users should be informed that they may need to format SD card to FAT32 before flashing (for G8142).