Unfortunately, this is just par for the course.
Running professional email servers in business is an ongoing exercise in extreme risk management. There are so many moving parts if you don't have systems in place you end up getting fallout like this, and it changes all the time without notice or disclosure. Often at best its a guess, because these large providers do not tell you why, nor provide support when their feedback systems say nothing is wrong. Technically, its an FTC violation, for deceptive practices, but they haven't had any enforcement in years thanks to our congressional representatives taking a yearly salary of 200K+ and not doing hardly any effective work.
But I digress, You should have had your marketing segmented to a different domain from your transactional emails. Ideally a completely different server, but a subdomain works in the short-term.
This will likely be the quickest way to get things back up and running. Create a subdomain just for transactional, and start warming it up, slowly, (i.e. send mail to an account you can check, open the mail, they record these clicks for metrics, increase the volume gradually, make sure you have rate limiting; per day and per hour, by recipient base domain). Ensure there are no mis-configurations by checking with a service like Mail-Tester. Then start pruning your email list for people that do not respond an affirmative after a set time. Ideally you should be sending them at least one email every few months to confirm they still want to be receiving emails. Any non-response should drop them from marketing.
Additionally Google and many other large ESPs fight spam by testing against whether you are in compliance with the workgroup whitepapers that are regularly published. There are a lot of them, and they also weight you worse if you have drastic changes in volume.
The whitepapers can be found at M3AAWG.org.
One of the most difficult to diagnose issues from a senders perspective are spam traps. Google will automatically mark as spam internally if you send to one.
Not only that, mis-configurations can get you flagged regardless of what you do, for example like not having a correct PTR record (reverse dns lookup) because your ISP changed it without telling you.
Yes I've seen this happen at some local ISPs, and most of the people I had to go through to get that fixed were completely incompetent with no guarantees they wouldn't do it again. In those cases with little choice for ISPs, there isn't much of a choice but to set up a load balancer to run everything through in a data center that does provide that level of guarantee. Cheap data centers plans or services won't, and may block those ports. Colocation usually is more expensive, but if they touch your equipment there is legal remedy.
A spam trap is some email address that has been disabled for a month (rejecting email), and then turned back on. If this is your problem you are not properly managing your subscriber lists.
You may also be penalized more if your mail being sent by your server is not including both TXT and HTML (having just one is often considered a mis-configuration), or does not support list-unsubscribe.