In theory, they can't share ad targeting data. That said, with the DMA they should not be able to share "data to effectively make you able to complete tasks", that's the point. Interoperability between Gmail and YouTube should equal the interoperability between Gmail and Peertube or somesuch.
The loopholes are ridiculous, not to mention that they are obviously going to just break everything for you if you don't comply anyway, but there's so many things they could do to throw sand in the gears- what matters is that the EU is doing something, it's reacting, it's trying to fight it and keep our common European values and not let foreign corporate empires dictate how things are done here, futile as the prospect may seem.
An efficient solution would to just stop using Google's services all together. That way they can't spy on you so easily and make a profile from your internet habits.
There are plenty of alternatives to Google services in the opensource realm.
De-googling is not a simple task, it takes actual man-hours of proper work to be done, especially when we're talking decades of accounts et c. Even when and if that gets done, what are you going to do about Android? And even if you sort that out, there are a hundred other digital ware that would need the same treatment.
At some point the task becomes insurmountable, it seems, not to mention you already need to have a solid technical background to have a chance of pulling it off.
I would love some suggestions if you have them, but frankly I am getting too old for this shit, I've fought it all my life but I'm just the one person and my time is limited you know what I'm sayin.
Does Google allow de-linking between accounts that were linked previously, then? I've seen people holding into a Google account just because it's keeping hostage other accounts they actually care about.