Do it: https://developer.apple.com/documentation/safariservices/safari_app_extensions/injecting_a_script_into_a_webpage

Yes, it’s possible on iOS, at least for mailto: links clicked in a web browser.

Just make a Safari extension injecting a .js script that modifies the DOM, changing all mailto: links to airmail: links, and you’re done.

Such extension would require few lines of code and could be developed too for other popular browsers on iOS, like Chrome.

This is not currently possible in IOS until Apple allows this.

That's an iOS restriction. Nothing ANY developer can do about it until Apple adds the capability.

This is possible on Desktop by checking the general preferences in mail, unfortunately iOS does not have the capability of setting default apps (only allowing apps to have defaults they can use in the future.. nothing system wide).

I'd love to have it, but this is an apple feature request. Please raise to apple feedback :D