I am the tech lead on the Flywheel proxy.

Flywheel does not MITM SSL connections; it does not proxy SSL at all. If you just mean "MITM" in a generic sense, the fact that you believe you have "data sovereignty" is interesting since we're talking about unencrypted HTTP here. Nearly all ISPs and mobile carriers undertake proxying and extensive analysis and manipulation of in-the-clear HTTP traffic. We agree that users need to opt into this feature -- since you have to trust Google to proxy your traffic, after all -- but it's important to keep in mind that many other parties on the path between you and a website already do transparently proxy your unencrypted traffic.