The issue here generally boils down to the defining difference between a generalist Admin and a Messaging Admin. The generalist can follow instructions, and nearly all the instructions out there stop at the point where SPF/DKIM/DMARC are successfully implemented. A generalist worth their salt will then fill in the gaps if they can', and knows this isn't where you stop when you want mail deliverability. There's a higher bar.
If you follow instructions written by non-professionals blindly you don't ever reach the point where you get to quality work.
Google, Microsoft, and the other large ESPs don't refuse to relay your email based on secret internal factors. This is what the non-professional people say to falsely justify why they can't do something.
Google and Microsoft publish the internal factors they use in the form of whitepapers at the industry working group. Its not ready made, and there are a lot of them, and they may not release their specific implementation details, but the metrics are there and often are based in weighted form (reputation-based systems).
If you follow them correctly, and set up the appropriate reporting accounts, and maintain those accounts, you won't have these problems. You generally only have problems once you've violated guidelines continuously, which happens when you rely on, or are unable to discern between qualified and unqualified help.
The factors are published at https://www.m3aawg.org/
Every professional that specializes in email or messaging that I know of is well aware of this.
People don't have the same vitriol when it comes to comparing Generalist Admins to DBAs, and this is the same with any specialized niche.
If you need email and messaging to work in a complex environment, you hire a person that specializes in it.