FIXED IT! I tried instead of the official AV adapter, a high bandwidth USB-C to DisplayPort cable. This worked instantly with full res at 120hz. I got the "WARRKY USB-C To DisplayPort" cable on Amazon UK but im sure any high bandwidth one will work.
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Exact same monitor (IIYAMA G-MASTER GB3461WQSU-B1) and problem and can confirm SwitchResX works but its unable to push past 60hz. This is with official USB-C AV adapter and a good high bandwidth HDMI 2.0 cable.
Monitor also recognised incorrectly as PL3493WQ.
Im going to try a UBS-C to DisplayPort cable and report back if it helps.
Thanks so much joea, I had this stuck on beta 1 and signing out if iCloud fixes it.