[Xiaomi Mi Mix 2S ROMs, Kernels, Recoveries, & Othe] EChondo:
*Edit2; Second edits are after updating to the 4th Nightly Build if anything has changed or further testing was done on the 4th Nightly.

Just updated/upgraded my Mi Mix 2S last night. I was originally running ResurrectionOS(?) and was quite happy, but was sad to see that the OP of that ROM was no longer providing updates(phone was damaged). I was debating going unofficial for Lineage, but was just lazy. Finally saw that it's official now!

Since ResurrectionOS is based off of Android 8.0, I needed to update to an "official" Android 9.0 Xiaomi build first, so I had to;

Backup data off of phone

Download the latest Developer Build(NOT stable!) from Xiaomi(literally just Google "xiaomi mi mix 2s download", should be 1st or 2nd link, then select the developer build download).

Fastboot into TWRP then Wipe>Format Data and then back to main menu and Wipe>Advanced> Select Cache and System then Wipe.

Reboot phone and then Fastboot into TWRP again, then transfer Developer Build ZIP to phone(I manually copied the zip over, I believe you can ADB Sideload but this did not work for me), then Install the Developer Build with TWRP.

Reboot into OS(Developer Build), do basic setup(no wifi, no google account, etc.) and enable USB debugging via Developer options.

ADB Boot into Fastboot, then Fastboot back into TWRP.

Once in TWRP you can follow LineageOS's guide for the Mi Mix 2S OR what I had to do;

In TWRP, Wipe>Format Data and then back to main menu and Wipe>Advanced> Select Cache and System then Wipe.

Reboot phone and then Fastboot into TWRP again, then transfer LineageOS, GApps, etc., then install LineageOS first, then GApps, then whatever root/apps you want.

Reboot phone and it should load into LineageOS from there and you can do your full setup.

Some issues I ran into when installing;

ADB Sideload did not work for me whatsoever in TWRP. I downloaded TWRP from LineageOS's website, however my PC(running Linux Mint 19.1, connected to phone over USB-C to USB-C) was complaining about udev permission issues. I wasn't experiencing this permission issue in any other step, so I went with manually transferring the installation files and then doing the Install on TWRP.

Official Stable build from Xiaomi did not work. Also flashing just the vendor.img, system.img, and/or boot.img did NOT work. Had to Install the full developer build. I am assuming this is because I was on the older 8.0 firmware and required a full and complete Wipe and Install and setup.

I experienced issues when I "fastboot flash recovery twrp-x.x.x-x-polaris.img"(command straight from LineageOS's guide) as my Recovery. I would get stuck in a bootloop going back into TWRP. I fixed this by doing a regular default Wipe swipe(just go into Wipe and slide the bottom slider) in TWRP after installing the Developer Build. I then was able to boot into the Developer Build without issues. So that's why I'm fastbooting constantly into TWRP throughout my process instead of doing the physical button combination.

Some issues I have had/noticed running the 3rd Nightly build from LineageOS;

Notification light is...finnicky? It lights up on some SMS/MMS messages, but others not, even if from the same app(Messages, Twitter, Discord). This isn't a huge deal for me, just something I've noticed. *Edit2; this is still persisting. I got a text message and did not touch my phone for 3 minutes and no notification light was emitting. All settings for the notification light are on and Messages app is showing everything enabled via it's Notification settings menus.

The "swipe down to exit fullscreen image/video" gesture does not work. I can swipe up to bring up the software buttons and go back/home from there. I know this gesture is supposed to be available since I received a tutorial pop-up from the OS on how to do it haha. *Edit2; Tested further, I do believe this is still broken. I am able to "Swipe Down" on fullscreen previews in 3rd party apps(Snapchat, Twitter, Instagram), but in Google apps(Messages) and OS app such as Files when previewing pictures.

I use Dual SIMs with the phone, Verizon and T-Mobile. I have noticed that Mobile Data is enabled on both SIMs at the same time, however only one SIM is truly LTE active. ResurrectionOS only allowed one SIM to be Mobile Data enabled at once and again only one SIM was truly LTE active. I am assuming this is a visual bug with LineageOS as it is behaving the same as ResurrectionOS. Though I am unsure if both should be active as I to my understanding this isn't a true Dual Active, it's a Dual Standby phone, right? Either way doesn't effect me, just something I've noticed. *Edit2; I can confirm for my phone that only one SIM can be Mobile Data active at once. VOLTE settings do not change this. So currently Dual Standby, NOT Dual Active. I'll see if this is just an issue with my phone specifically or not, but will try to report to the devs.

I do see a 4th Nightly was pushed. I have not tried updating as I do not want to be wiping my daily carry every night haha. I'll see what the Nightly update process is like and then decide if it is worth it(certain fixes have been provided).

*Edit, this is my first time on LineageOS and I just found the Updater! Hahaha. Going to update now and see if anything is fixed and will update my post if so.

What does work(tested so far) and what I've liked;

Bluetooth/Pairing(both for Headsets, headphones, and to my car(JVC KW-M740BT)) Work. Have NOT tested Android Auto yet.

Calling and SMS/MMS texts work on T-Mobile and Verizon out of the box, however Verizon does not know what this phone is, so it limits MMS picture sizes. So group chats work, however picture MMS will be downgraded. This was the same experience on official and ResurrectionOS builds.

Battery is sameish as my old build. Did some calls(hour long), texts, web browsing and used Waze for a 1 and a half hour commute(screen on whole time at max brightness), and I'm down to ~50% after 12-13 hours.

Phone seems a bit snappier, though it may because of the factory resets and the jump to Android 9.0. Clean slate and all you know.

I am loving the all blackish/gray status bar menu and settings app. And I love that you can change the theme to "Black" and it will be pure black menu items. Also love the accent color options, gives some personality to the OS.

I read previously that some people had issues with charging? I am using a Monoprice USB-C to USB-C cable and a Monoprice wall charger. I have not noticed any "Slow Charging" notifications, so charging seems to be working as expected for me.

I installed the "Stock" GApps package, which still has some old builds of apps in there. I had to let the Play Store update all the apps overnight. So Google Camera was on an extremely old build at first and was simply crap the first night, but the update overnight provided the latest build and it is just as snappy as usual now as well as other various apps from Google.

I am using the LineageOS launcher. The Pixel launcher was installed via the GApps package, but I have not once used it. The LineageOS launcher is simply perfect and gets the job done.

Overall I'm pretty happy with the switch. I hope my post helps anyone here that may have ran or is running into the same or similar issues.