[Samsung Galaxy S9+ Guides, News, & Discussion] ianmacd:
I ended up installing Soldier's excellent stock+ ROM in the end.

I appear to have been very lucky with my particular S9+. Installing it has been much easier than anticipated, given the reading I had done prior to the device's arrival 2 days ago.

OEM Unlock was available as soon as I enabled Developer Options after unpacking the device and turning it on. I toggled it, accepted the factory reset, and when I booted into download mode, both OEM Lock and FRP Lock were off, and there was no mention of RMM. (I've noted this before, but I'm just recapping, because I can finish the whole installation story now).

I then installed TWRP according to the procedure outlined in the device's TWRP thread, i.e. including the no DM verity/no encryption patch.

Then, I spent a full day researching which root method to use, because there's currently a lot of contradictory information circulating regarding which methods work and which don't. For every possible combination of phone state + stock/custom firmware + stock/custom kernel + various ZIP files to be flashed to aid rooting, there are people who claim that this particular combination worked for them, and others who say that it didn't, and in many cases landed them (back) in frustrating 7 day lock status. Usually, rooting is quite trivial, but there are a million variables in the case of the S9+.

I usually run stock ROMs on my Samsung devices, rooted with SuperSU, but there were just too many reports of SuperSU behaving very erratically. In addition, SuperSU development has stagnated since Chainfire retired from the project, so I was already pondering the wisdom of continuing to use it.

In the end, Soldier's ROM seemed like the simplest solution. It's very close to stock, but has all of the anti-Samsung sanity preservation patches built in. I went with the custom kernel installation option, and rooted with Magisk this time instead of SuperSU.

The device is now working like a charm. TWRP + root (inc. passing SafetyNet) + no waiting for OEM Unlock to appear. I can't believe the misery I've been spared, compared to some others.

It's taken me 2 days to root it, because I've ploughed through close to 100 pages of XDA threads today. If I hadn't had to do that, I could have been rooted within an hour of opening the box.

I just don't understand why there is so much variation in how these devices behave out of the box. Some people have OEM Lock straight away, but others must wait a week. Some people can toggle the setting without a factory reset, but others can't. Some people who perform that reset are then faced with prenormal RMM status, while others aren't.

One thing that might be significant in my case is that I never logged into a single account on the phone until the device was running Soldier's ROM and fully rooted. Nor did I install a SIM card in the device. In fact, it has still never had a SIM card in the tray even now. I'll move my SIM card over from my S8+ tomorrow, once I've got the phone fully configured and all of my apps restored from back-up.

Anyway, I just wanted to share a concrete story of how painless the installation of this particular device can be. I just hope that mine doesn't now start to exhibit random weirdness in the coming days.