I have had the benefit of multiple/external monitors at (eg banking) client sites in the past, but I don't really miss them, and often forget to use them even when available to me regularly.
Laptop ergonomics are indeed far from ideal. Possibly the biggest help for me was to get my varifocals set up to be mostly central area optimised for my typical laptop screen working distance, with eg much less 'far' vision at the top.
I arrange my workspace to minimise the bad aspects, eg having the laptop at the back of the desk to minimise the angle down to the screen.
A 4K monitor is certainly an improvement, tbf. But I can be productive without one.