How to recover glitching Galaxy S9? The bad
Last week I totaled a Galaxy S9's screen which I owned for over two years. I liked the phone so I headed up on Facebook market to look for another one.
I discovered a phone that I liked and went on to buy it.
When I had the phone in my hands, before paying, I run a service code (#0011# to see if all sensors/screen etc worked). This passed so I bought the phone.
Also, the phone won't do Samsung updates. It would either throw an error after download (The downloaded file is not correct) or restart when attempting.
The ugly
Here's the harsh part: not long after leaving the dodgy seller, while going home, the phone started behaving really weirdly: sudden restarts, some screen glitches, and artifacts. The phone also loses battery pretty quickly, either due to the weird sudden restarts or else.

Note the purple glitches. I was able to record these with a screenshot, meaning it comes from the GPU chip?
The good
The first thing I looked at was the water damage sticker beneath the sim tray:

I then managed to charge the phone fully and log in with a Google account, which allowed me to do further diagnostics. I installed Geekbench and run a test. The phone has a normal score for this model.
I tried filming, calling, playing games on the phone and all seems well.
The only times when the phone restarts are after receiving an error like &quot;Google Services App crashed&quot; or &quot;Samsung Services App crashed&quot; or suddenly when I try to use a service or disable/uninstall blootware.
I also managed to get the phone to update through Samsung Smart Switch to the latest update (Security Patch 1 Dec 2020)
Resolution:
I have a few options for repairing this weird phone

Find what version of firmware I could flash on the phone and hope for a stock experience (including bootloader etc) - This option assumes that the hardware is intact, the chipset has no water damage, etc.

Open the, in which case I lose IP68 (assuming this is not lost already) and change the motherboard with my old phone's.


How to proceed from here?
I am a bit unfamiliar with choosing the right firmware etc. I have the following information in the phone's software info:

One U.I 2.1

Android 10

Baseband version: G960FXXSBETG8

Kernel version: 4.9.118-18847185 # 1 Mon Aug KST 2020

Build number QP1A.190711.020.G960FXXSBETH1

Knox 3.4.1 API level 30 TIMA 4.0.0

Service provider SW ver.: SAOMC_SM-G960F_CKH_DRE_QQ_0009 / 2b282a7f23017ece (DRE/DRE/DRE)

CSC: G960FCKHCFTK1

AP: G960FXXSCFTK2

CP: G960FXXUCFTJ1


I tried downloading this firmware version, but from what I've read the bootloader version here is too old and cannot replace the current bootloader. How can I determine the bootloader version on my phone? XDA forums say it's the 5th digit from the Build Number. However, my Build Number doesn't seem to respect the format of the above file.
Can anyone suggest what to do next? I'm itching to open the phone, but I think it's wiser to try and repair it from firmware.
bootloader level is increased in 1 digit
1 2 3 4 5 6 7 8 9 A B C D
G960FXXSBETH1 = B
G960FXXUCFTJ1 = C
you can download rom for SM-G960F from updato, samfirm, frija or samloader. try lowest bootloader first, as you already stated downgrade is impossible (and higher version usually comes with higher protection against rooting). you can check bootloader version from download mode screen