IMHO, the real issue with Google isn't the control of the web browser, but the complete vertical integration of the ad space. Divest Google of its position as the ad broker (i.e., unwind its acquisition of DoubleClick), and prohibit any sort of privileged data flow from Google to ex-Google Ads, and you'd probably have a more viable solution than the poking DoJ is trying to do right now.
Why? Google develops ad-supported software, they run their own ad placement service for it, and they allow others to use that service to run ads on their own sites. I think there's a bit of a conflict of interest between running the website ranking service and running an ad service that ranked websites can choose to use, but that's minor compared to also controlling the browsing platform.
If Google started ranking sites that use AdSense higher than other sites, you could probably prove that in court fairly easily — your lawyers would make them hand over their ranking code during discovery, and it would say `if (usesAdSense) score++`, and you'd win.
But if they made (are making) subtle web platform changes via their control of the browsing platform that hurt competitors and help Google, that's extremely difficult to prove. Look at third-party cookies, for example.
This first one is about dominant market position by paying for placement on all surfaces (iPhone, Firefox, etc.) plus chrome. So splitting off chrome and android make sense in the context of the suit.
With the Privacy Sandbox Google already prepared for their sell side ad business to be sold. With the ad auctioning logic and data flow being handled directly in Chrome PS they wouldn't need GAM anymore and would probably easily accept this part to be split. And it could even reinforce their vertical integration.
On the other hand with no direct access to the browser and no 3rd party cookies anymore, they would probably be less relevant for media buyers on the open web, which could open a good chunk of the market for other ad platforms.
You have to wonder if Google didn't somehow maneuver the DOJ into telling them to do something that's going to result in higher costs and user inconvenience, as opposed to something effective like you've proposed here.
On the other hand, Google would argue that Doubleclick is Google, and everything else Google does is just to create channels for ads, and acquire data for ad targeting and pricing.
