On Mac, hold down the "c" key and a little menu with accented versions (ç, ć, č) appears, with numbers to choose which one. The flip side is that holding down letters or numbers doesn't repeat them on a Mac.
I don't have one anymore to test though, but I used to use the mac layout on Linux for a while exactly for composing using alt.
Or maybe this has changed (back) in a recent OS update? I'm running 12.0beta on this machine.
The dead-keys like Opt-E for acute only work with a small set of letters in the standard US keyboard layout, although it's possible for a different layout to support more -- subject to the combinations existing as precomposed characters in Unicode. (For other combinations, you'd need to enter a combining accent after the letter, and rely on the font to supports placing it properly.)