It also depends on the reputation of your neighbours / AS... try to setup an e-mail server with a cheap VPS in DigitalOcean|OVH|Hetzner and even if you kept using the same IP address for 10 years I don't think you will be able to relay to gmail
2007..2014 were probably the worst. Gmail was chainging often, Microsoft was blocking everyone.
I think self-hosting is easier now than 10 years ago.
This is because the business department will try to force admins to ignore requests to remove spammers as the spammers generally pay decent money.
By blocking everyone, it forces the ISP to get off their ass and kill the spammer irrespective of how much they are willing to pay.
https://blog.sucuri.net/2021/02/uceprotect-when-rbls-go-bad....
ISPs are now so sensitive to not allowing spammers that the RBLs can't make money from it anymore.