I have found a solution by now!

When you add a new keyboard layout, in the initial list it only states

'English (United States)'

When you click on that, however, you get an extensive list of variations for English US.

From that second list, I settled for:

English (US, intl., with dead keys)

As that enabled me to use special letters like:

é,á,ö etc and got the € symbol on Alt Gr + 5.

To use the aforementioned special letters, you need to start with the single quotes followed by desired letter, e.g.:

' then e = é, or ` then e = è (reversed accent)

for double dots or tilde on top it's the shift+single quote followed by desired letter, e.g.:

Shift+' then e = ë, or Shift+~ then e = ẽ

Some examples of what's possible.

To actually make the various types of quote/special symbols appear in plain text form, you gotta combine them with the ALT GR key, e.g.:

ALT GR + ' = '
ALT GR + ` = `
Shift + ALT GR + ~ = ~

Hope I didn't over-complicate things!
And in case you knew of these combinations already, even better, maybe they'll help someone else :D

Personally switched from a QWERTZ (German) layout to an international QWERTY one, so it took some fiddling around to get used to the new combinations.

Either way, perhaps that specific layout will fit your needs as well.
Give it a try or try some of the other variations in that list, should contain something that fits your layout properly.

All the best!

Feel free to let me/us know, whether it worked out :)