Here is kind of rundown of steps I've done (before renaming twrp to twrp.img)
1. I was originally on HavocOS (Android 9 based), I flashed Android 9 with fastboot.
2. Flashed LieageOS after Android 9 (made sure stock firmware boots before flashing Lineage)
3. At this point I couldn't do fastboot boot, so flashed stock firmware again through fastboot booted it and let it update to stock 10.
4. With stock at version 10 I could fastboot boot, stock FW link in OP is broken (at least for me)
5. Went back to stock FW 9 through fastboot, then flashed FW9 again through system update -> local upgrade
At this point I renamed twrp to twrp.img.
1. flashed LieageOS, and still have fastboot boot option
2. Tried rootfs method, but that didn't work (I think I need to run as root, I'll try again later)
but want to check something:
In case of rootfs method, do I still flash halium-boot.img? Or does halium-install takes care of it?
And finally sometimes (happened twice) phone gets stuck at Booting screen (can't even access fastboot at this point), is there something specific that I can do about it? Both times I got it working after sometime just pressing volume and power key