I was thinking of DPI for text is all but maybe I don't 'need' 4k. I'm not really sure what DSC is though.
Yeah, that makes sense! 4k @ 27" does look insanely sharp for text, much more so than 1440p, especially with how text actually looks on a 1440p OLED. It's one of the main reasons I keep coming back to 4k @ 27" because the text clarity is so so good.
DSC is Display Stream Compression, it's considered
visually lossless and helps reach high resolutions/refresh rates where the bandwidth on an existing port would be exceeded. You can end up in a rabbit hole here with some people thinking it does make a minor visual difference, others noting input latency changing because of it (although nothing really ever proven) and then a large majority not really caring either way.
It's more noticeable now because we've got higher refresh rates at 1440p and at 4k - and some of the 'ports' on the monitors aren't using latest spec bandwidth. DP 2.1 supports 4k 240hz without use of DSC - so you could essentially run 3x 4k 240hz monitors off your 5090 without any issue provided the monitors were also DP 2.1. But the max from HDMI 2.1 (without DSC) is 4k 120hz (at 10 bit - there's a whole extra layer to this depending on what colour depth you run).
I ran a 240hz 1440p monitor on DP 1.4 at 240hz without DSC because I was using 8 bit colour, not 10 bit - if I swap to 10 bit the max supported resolution available to me was 200hz.
If your monitor only supports DP 1.4 (and it does vary panel to panel how each manufacturer implements this) you may find that plugging into a 4k 240hz monitor would end up using two 'display heads' on your GPU - so you'd essentially be using 2 video output ports (not physically) on your GPU for a single monitor. It differs from GPU to GPU and monitor to monitor, it was a lot worse on the 4000 series from Nvidia but I'm not running enough monitors at a high enough resolution/refresh rate to test how it works on the 5070 Ti.
If you wanted 4k OLED monitors at 27" for the side monitors then it's probably worth considering looking at a monitor that lets you disable DSC (Asus are good at this) so you could run them at 4k 120hz (no real benefit of running a secondary monitor at anything higher) and then only use 2 of your display heads on your GPU. That would mean your main 32" 4k monitor could happily only be on DP 1.4 and use two display heads at the full 240hz.
Aware this is a very long response... sorry !