[Galaxy S6 General] waffelschnitten:
Hello,

as I have installed a custom ROM myself on my S6 G920f only a few weeks ago, I can help you out as far as my knowledge goes and as far as I haven't forgotten it. Bear in mind though that I am not an expert and had to look up a lot of stuff myself.

First, after trying many different up-to-date ROMs and after becoming very frustrated, I kind of accepted that there is currently no up-to-date ROM for the S6 that does not have one or the other big flaw. Some didn't even Boot and instead did load for 5 minutes and rebooted into twrp. The critical flaws on the others were random shutdowns, microphone issues ranging from my voice not being transmitted at all to constant feedback where the other person was hearing her or his voice. The feedback issue I experienced on all the ROMs. Also, with the ROM that I thought I could arrange with I noticed that sometimes, randomly, the calls coming in were just blocked/didn't come through for whatever reason. Also, fast charging is an issue and the camera is bad since you won't have Samsung's camera app and the necessary framework that optimizes it. Google safety net was a big issue with the Android 10 ROMs too, but magiskhide props solved that for me.

I'm sure many people have different reasons to use ROMs but for me it was just not worth it at this time. Maybe this will change later. Just wanting to let you know since you asked for it. I found my current happy place with the Marduk Stock Rom which you can find in the S6 Development section. It is based on the stock ROM which is Android 7, I find it to be fast, it has a good battery life and for me it is very stable. Also no microphone or connectivity issues, fast charging works only with screen off, camera is good since it's the original camera app. No google safety net issue.

Now to your questions.

Debloat means the pre-installed apps that you cannot uninstall and that clutter your phone are removed. Deknoxed as far as I know refers to Samsungs knox which is a safety plus as long as you use your phone unrooted but (tries to) restrict rooting and installing custom ROMs etc. Don't know much more than that tho.

Yes, your bootloader seems to be unlocked.

Now install twrp first. Go to twrp website and download the version you need for your phone. To flash TWRP recovery, you don't need odin. I have used heimdall myself and found it to be very easy.

- Install heimdall. You can also install samsung usb driver for linux if you have issues connecting your phone to your pc.

- Boot into Download Mode (For me volume down + home button + power button)

- Have phone plugged in to pc and at least 60% charge

- Start Terminal, Cd into folder with your recovery.img and use this command to install your recovery (of course replace "recovery.img" with your recovery):

heimdall flash --RECOVERY recovery.img --no-reboot

- Now turn your phone off, and since my phone automatically rebooted at this point, try to stop this by instantly using your key combo as soon as the screen turns black to boot into recovery mode (volume up+home button + power). I had issues here. If you don't manage to do this and get stuck in bootloop just boot into download mode again and install recovery once more to try again.

- Now you can download your preferred rom, boot your phone into twrp recovery and connect to your pc to copy, wipe everything in twrp (obviously save any file you'd like to keep) and install your Custom ROM.

- Most ROMs (but not all! read ROM description!) come without GAPPS installed so you will have to download and install it manually after you have installed your ROM (I recommend GAPPS pico for your custom ROM version). You don't have to do this if you don't want to use google applications.

- Before leaving twrp and booting into your OS install Magisk in TWRP(Download latest version and install in twrp after GAPPs).

- Wipe Cache/Dalvik

- Now reboot into System (and don't install TWRP manager system app)

- Install Magisk Manager after after restart

ROMs:

The best LineageOS ROM I have found to be this: https://lineageos.wickenberg.nu/zerofltexx

It seemed to have the least amount of flaws. The bugs for me were no fast charging unless you reboot plugged in. So no fast charging when you plugin and dont reboot and stay plugged in after reboot. Random connectivity issues. Microphone feedback. Camera crashes and camera issues whilst videocalling etc. Google safety net failure by default, but you can fix this with MagiskHide Props.

Haven't tried the Pixel one here but it seems to have the same microphone issue where nobody can hear your voice.

The Marduk Debloated Stock ROM, also here on XDA, is based on the older Android 7.0 but works very well. Lacks the extended settings and features you find on LineageOS, but least buggy and fast and battery efficient.

Have fun