Earlier quoted context omitted.
Multipath-TCP is designed for precisely that.
I may be wrong but isn't Multipath TCP pretty darn new and rarely used? At least on iOS you have to explicitly opt into it, either using the new Network framework for raw networking or a special configuration for URLSession, and also requires an entitlement to even do (no idea why). AFAIK the only Multipath TCP that my iPhone regularly actually uses is Siri. Which is to say, it still feels largely experimental.
I just mentioned it as it's a cool project and in time will help address some of the limitations you mention.