That suggests a simple solution to the problem that can be done on the gmail side: for any small mail server (to pull a number out of my behind, say less than <200 emails a month to any @gmail address for the last 12 months), white list them if they satisfy the rest of the usual requirements.
Even if someone attempts to game the system and creates a number of servers, for an effective spam campaign it means a large number of servers, costs go up.
It might be worth experimenting with "abusing" this behaviour to put your small server on the Gmail whitelist. Start sending a large number of generated emails to a @gmail.com mailbox, log in and ensure none of it ends up in spam (ideally automate that too :), and there we are. Anyone have any idea how many emails that is? :)
> In the old days, if I didn't receive mail because my mail provider was rejecting it, it was me who had to talk to them. Free has a cost.
In the old days average email user was being inundated with low-quality spam and getting mired in fraud. Google fixed that problem for their end users. In doing so, they made it hard for a small % of people who want to run their own email servers, but they've given all of these people an out -- go sign up with someone who knows what they're doing, and google is going to effectively outsource the fraud/spam management to those companies.
You can tell me I have it wrong but I'm literally just describing the landscape that actively exists and how to navigate around it for anyone who wants to, but you're not taking out google.