It sounds like you hit the point in Linux where this is painful: mixed DPI displays with mixed display scaling. Wayland fixes that. And soon, we'll finally have fractional scaling, too. GNOME is close to delivering that. So, it's getting better.
This does make things a bit too big on the 24" external display which has nearly the same resolution, but it's not distracting. Some websites I use CTRL-+ and CTRL-- to change the scaling, or rather fine tune it, depending on more than +-20% and the rendering gets wonky. Firefox seems to remember this per site even after a relaunch or reboot.
The biggest problem I have is Bluetooth, my mouse is always disconnecting on Linux, dozens of times per day, but the same laptop+mouse rebooted to Windows 10 - no problem.
I thought it was odd when Apple got a lot of heat for not using X and writing their own thing. It sounds like Apple had evaluated X and determined it wouldn't fit their needs. The same thing happened when things like Wayland started getting written.