I recently bought a 240 Hz OLED monitor as well, but to my dismay the 80 Gbps data rate mode is actually optional in DisplayPort 2.1a. Even though I have a DP2.1-capable GPU (9070 XT) and monitor, it's not possible to do 4k 10bit HDR at 240 Hz with DSC off. Since I don't want compression to be on I've compromised to sticking with 180 Hz, which as you say due to the diminishing returns above 120 is still plenty.
Still quite frustrating that the display industry did it again in specifying a standard that makes most of what's interesting about it optional, so everyone can print it on their boxes without delivering the expected value.