Asklemmy

On Linux/Unix (not sure about other platforms), you can configure a "Compose key" (people usually use something like the Menu key, or the right Alt key).

You can then hit that Compose key, followed by multiple other keys and they will be combined into a character, usually roughly relating to the shapes.

• Compose L / gives Ł
• Compose + - gives ±
• Compose 1 2 gives ½
• Compose C = gives €
• Compose - - - gives —
• Compose c s gives š

etc. There's a big table somewhere where all these are defined.

On android keyboards you can hold down the letter and get a list of "alternative" letters (seems to depend on the keyboard and keyboard layout configured what you get), you can also configure multiple languages/keyboards and then switch between them, by holding the spacebar.

If you're using Windows, you could look into Microsoft Powertoys, a suite of cool extensions which adds some nifty functionality.

One of them (I can't recall its name) allows you to quickly call up a toolbar for special accented characters, and other symbols - eg, if you want the © symbol, you type C, activate the toolbar via a keyboard shortcut and it's there to select along with ç and other relevant options. I use it all the time, it's really handy.

EDIT: Here's a link, and that tool is called Quick Accent

How many keys would that amount to if each one actually had a button? This page makes me think it would be far too many.

vae victis latina clavis

On android you just hold the letter ߧśŝšṣ̌şșṣ (this is all the s' i get.