Believing Google values privacy over profits is a bit naive I’d say.

Nevertheless it could be strictly better: the new version gives the OPTION to share less data, whereas the old version does not really give you the option. If you include third party scripts, they can just send the information to third parties directly.

Google could also be audited and would then have to prove that they really didn't share ay data, or whatever.

In the old way, site owners who did not want to send data to 3rd parties could just NOT insert the "send data to 3rd parties" code into their website

Now they still have that option, but users have no technical means to determine whether this is happening

In the old days, servers could still share data with third parties, so in that sense, there never was any way to be sure.

the big difference is that I cannot audit the websites using this new version of GTM (and I don't trust marketers)

This method of the server sharing data with third parties was already possible before.

Sure, just that Google didn't propose it

So far they only propose their server proxy thingie. Is it clear what data they intend to share?