Good suggestions, I already had ReBar enabled and had a look at RSR but you got me thinking about resolution so I experimented with that after you mentioned it.
I found that the problem is that Minecraft RTX doesn't seem to support how wide my monitor is. I'm using a LG 40WP95C-W (pretty much 21:9 Aspect ratio).
I tested this by going windowed mode, you can see where the issue starts appearing.
As wide as I could go manage before the issue occurred
A Fraction Wider Screen - Obvious rendering issues and smearing - Unplayable
Full Screen - Obvious rendering issues and smearing - Unplayable
But good news, I can still play with pretty visuals, just not UltraWide.