The current situation seems unimpressed.
Unfortunate indeed. Time to look for new phone.
What STOCK FW can I flash on the S9+ (SM-G965N SK version) in order to be able to flash TWRP and root? I’ve heard horror stories on newest FWs that “block” download mode where you can’t flash TWRP?
Nothing in the post suggests that a device with a fully unlocked bootloader will start to suffer from Prenormal by means of a reliable “latest” firmware upgrade. Follow the guidelines for the device, eg https://doc.e.foundation/devices/star2lte.
The solution for the OP involved an exchange of phones ! He was advised a sane path to take with his particular replacement device. (My guess this was a grey import device imported as Oreo, but the OP upgraded to Andriod 10 (more securely locked now) and a paid for service advised that it now would not unlock.) Other join in but we are not given full background details of their full situation.
Now specifically to you. Do you plan to install /e/ on this Korean model and are you planning to operate it in South Korea? Is the device showing RMM-Prenormal now? (Insufficient information provided.) If it is at Android 10 and a grey import then RMM-Prenormal might welll be a problem. If at 8, upgrade probably inadvisable.