You can use better dummy to achieve a similar resolution: See here https://github.com/waydabber/BetterDummy and this MacRumors thread https://forums.macrumors.com/threads/m1-mbp-lg-34-inch-ultra...

I tried it on my M1 Mini + LG5K2K and soon disabled it again. The performance impact was noticeable in Lightroom and the display adapter config got in the way once I switched display input to my work laptop.