Just a note that SPF/DKIM and DMARC are not "get out of jail free card".
Google support is just brushing people off with the link and they are somewhat right, because a lot don't have anything set. There is also a bit of black magic with spam filters that Google support will never tell you about because it will be just like SEO, someone will use it to pass filters.
That said, there are multiple other reasons that your email can be qualified as spam by the spam filter. Like sending out email from one domain and having links to another domain in the body as an example. Not having "unsubscribe" links on mass mail. There are legitimate reasons to throw email to spam because some companies get their infra or "contact us" forms hacked.
In the end users can be annoying because they might not understand what "this is spam" button means for the sender - they just think your mail was annoying and they did not expect it and press that and you are qualified as a spam.
Google support is just brushing people off with the link and they are somewhat right, because a lot don't have anything set. There is also a bit of black magic with spam filters that Google support will never tell you about because it will be just like SEO, someone will use it to pass filters.
That said, there are multiple other reasons that your email can be qualified as spam by the spam filter. Like sending out email from one domain and having links to another domain in the body as an example. Not having "unsubscribe" links on mass mail. There are legitimate reasons to throw email to spam because some companies get their infra or "contact us" forms hacked.
In the end users can be annoying because they might not understand what "this is spam" button means for the sender - they just think your mail was annoying and they did not expect it and press that and you are qualified as a spam.