Displays using MST to achieve higher resolutions than a single link allowed worked out more or less fine. The problem there is MacOS doesn't support MST.

Funny, I am typing this on a Mac connected to an MST Dell 5k display....MacOS supported this setup since at least 2015, when I bought my display.

That's a common misunderstanding. DisplayPort MST allows multiple display streams to be multiplexed onto a single DP link. So you are still bound by the bandwidth of a DP connection (25.92 Gbit/s for HBR3).

The 5K Ultrafine (which I presume you are talking about) is connected via two separate DP links which are tunneled over a single cable using TB3.

That sounds like nitpicking, but is a crucial difference. MST provides daisy chaining of displays, but can not help you exceed bandwidth limits. Whereas the two DP links are completely independent and theoretically allow you to double the max bandwidth. (well 2xHBR3 exceeds the max TB3 bandwidth, so the 5K uses 2xHBR2)

Aha! What MacOS doesn't support is MST daisy chaining for some reason, only MST to a single display to overcome bandwidth limitations.