Holy sh... I've been battling this for YEARS now on X11. I'll try Wayland and see if it's better
Some monitors also have unusual dpi where they are HiPDI but don't suit 2x UI scaling. A very popular LG USB-C 4K monitor is in that camp.
√(3840²+2160²)÷275 ≅ 16
For the external monitor to have the same dpi as the laptop, it would need to be a 16K resolution monitor.