Here is an answer close to 1st answer, with a little alternative.
I'm French using an US keyboard. So I often need to type é, è or à.
So like in 1st answer of this page, I use combo:
" then e for ë
' then e for é
` then e for è
` then a for à
~ then n for ñ (more for Spanish people)
These combo are always available in my configuration because I set my keyboard to English US international instead of English US or English UK.
With such keyboard setting, no need to type any compose key before the combo.
and these combo
" then spaceBar for "
' then spaceBar for '
` then spaceBar for `
~ then spaceBar for ~
Alt Gr plus 5 for €