The short answer?
@Stele88
Yes. To all of it.
The long answer starts with the Dirty Santa root process. Believe me, I was daunted by it as well, but as long as you follow it (or the noob version of the guide you can find by searching around the same section the original guide is in) and take your time, you'll be okay. It's what I did and I'm now where I am today.
I'll address a couple things since I have the same device as you. If you want to get your device working properly on LineageOS/ResurrectionRemix (LOS with more customisation) then it's a long process.
1. Start by following the bootloader unlock process for Dirty Santa. Depending on what firmware your device gets shipped with, you may need to downgrade to (in the case of the H990DS) the v10c firmware. Once done, then you can Dirty Santa the bootloader and get it unlocked.
2. Once that's done and you're rooted with the initial TWRP from the thread installed, you'll want to look into KDZWriter. It's a program by the same emdroidle who did the Dirty Santa process for us that allows you to update your STOCK system software to newer versions and keep root. You can use it for example with the H990DS firmware KDZ files and go from rooted 10c -> rooted 10f -> rooted 10xx -> etc. until you're on the latest available 7.0 KDZ for the H990DS. This will be the longest part of your process, but it ensures that everything is up to date before going to AOSP/Lineage/Resurrection, specifically the modem partition. I recommend using the TWN region of firmwares if you're just going to convert to a custom ROM anyway, but any region will work fine (the only real difference is that, for example, the Australia region of firmwares comes with more bloat than the TWN region).
2b. You can also use KDZWriter from an updated TWRP you can get in the thread by Phoenix591, his latest TWRP versions have KDZWriter built-in so you don't have to run it off the computer, you can do it all with the kdz files in recovery.
3. Yes, you can root using the Dirty Santa method on v10c kdz firmware. I recommend Magisk for root over SuperSU.
4. There's no official LineageOS that is as up to date as the unofficial ones, and there's no official ones with second screen enabled full stop. That being said, the unoffcial LineageOS 15.1 by x86cpu has been my rock solid daily driver for about 2 months now and I've never had any issues that stop me from using it. The same can be said for ResurrectionRemix 6.2 by Blaises. To use either of these, you will need the updated TWRP from Phoenix591.
4. Second-screen enabled builds of LineageOS and ResurrectionRemix do exist, and in this case ResurrectionRemix is (as of this post) more up-to-date than the LineageOS 15.1 second screen version. If you find yourself needing additional features that aren't in the stock kernels for those ROMs (such as KCAL configuration to help with burn in, among other things), then I maintain the second-screen version of the Gamma Kernel by Omar-Avelar. People are successfully using this with second-screen ROMs and have full functionality. You can purchase the app BoredSigns from the play-store to add additional functions to the second-screen.
As I said at the start, it is a long process, but a very worthwhile one. The jump from stock to custom-ROMs is absolutely incredible. Again, in short, yes. You can get everything you want.
Just do your searching around the forums for the processes I outlined above, ask any questions you have, pre-download your kdz firmwares from
HERE and any software you need, and just take your time. It's maybe a couple hours to get setup with a rooted stock system that's up-to-date, and maybe another hour or so to go from stock to Lineage/Resurrection.
P.S: If all goes well with the Dirty Santa process, but you get screen distortion after rebooting when flashing the TWRP from Dirty Santa, just pull the battery instead of rebooting.
Good luck, good searching, and remember to not be afraid to ask questions in the relevant threads.
---------- Post added at 04:50 AM ---------- Previous post was at 04:49 AM ----------
Too bad. Your earlier post actually got me excited. Hehe. Anyway, hopefully someones starts working on rooting the oreo firmware.
It's highly doubtful this will happen, but one can hope. Right now the best way to get Oreo is through a custom ROM, purely because of how security is implemented by LG in the stock Oreo ROMs.
