I use the french azerty layout (fr-pc) on macOS (equivalent fr-latin9 on linux). It is an ISO keyboard, I am able to do all of the examples you wrote, and more: option b is ß, ã is option ~ and a, "é" is its own key, so is "à" or "ç".
I am able to write 120 words per minute, not so special, but good enough for me. I find the ansi/qwerty too slow with its compose system (I tried on the canadian-intl layout made for Québec).
Is this a physical Azerty keyboard? How do you get around having to hold shift to type any number?
What are you in that your local ISO layout has these issues? The Irish layouts on X and Windows don't have this issue and makes typing just about any diacritic easy, though I go a bit further and use the UnicodeExpert variant. MacOS, OTOH, has issues unless I switch to an ANSI layout.
I was not claiming any local ISO has trouble typing other characters with compose key or similar.
My point was that ANSI is more ergonomic than ISO for programming, and all local characters are still easy to type.
A US ANSI keyboard has all programming symbols in the right place. Yet it makes Latin/Nordic and other symbols easy to type via compose key (e.g. Right Alt), or similar.
For example, Compose a + ' = á, a + e = æ, o + / = ø, s + s = ß, s + o = §, etc.
An annoying issue is that most brands won't sell ANSI keyboards abroad. Apple is one of the few that get this right. A trick is to import from countries where ANSI is the default, such as NL.