On mobile, it's pretty easy with a reasonable keyboard. For mine I just long press a letter and it let's me select the accent.

On desktop though? Yeah, it's a pain in the ass. I don't know how it's typically handled. I think it's possible to turn on a setting in KDE to let you press a modifier and then an accent and that accent will be placed on the previous character, or something like that. I'm pretty sure I saw that setting at one point, and enabled it, but I've never used it and don't remember how.