You don't say what platform you use, but this can be true of Windows and Windows-derived systems (which includes a lot of web stuff on all platforms because Netscape foolishly exposed Windows internal key codes 25 years ago). The non-alpha VK (virtual key) codes migrate all over the place¹ or worse disappear (e.g. Turkish doesn't have the VK corresponding to US +/= at all).
I used to work on Chrome OS which currently does this (trying very hard to emulate the Windows rearrangements due to the Netscape web legacy) but will shortly try the “what they say they are” method behind an experimental flag; that is, for example, the shortcut Ctrl+[ would be typed by Ctrl plus whatever you type to get ‘[’.
I'm not familiar with the low level mechanics of this but you seem to be. How would it work to do what you write at the end? Isn't altGr just an easier way of typing ctrl+alt? Getting ctrl+[ would then be ctrl+ctrl+alt+9. Or is it a proper key of its own?
In my situation I write in english, swedish, danish and sometimes icelandic as well as in programming languages. My solution has been to create my own keyboard layout that I call Nordic Programmer which is the US keyboard but by pressing altGr I have åäö letters where they are supposed to be, and øæ next to them. and then on altGr+eyuioadt i have éýúíóáðþ for icelandic. All of them are capitalized by adding shift. This was pretty easy to learn to use and makes me not have to switch keyboard all the time. It is not truly nordic I guess, because it prioritizes swedish which is the keyboard I learned growing up.