• change the keyboard language
• Not ideal, since your actual keyboard doesn't change
• memorise some pretty esoteric unicode strings; or
• use a virtual keyboard
Tbf, Macs don't handle all diacritics that well by default, but they do the most common pretty well.
Like Option then E gets you é. Option U gets ü and so on.
I guess that's a legacy of their prevalence in desktop publishing apps in the 90s.