I was in the very similar position, I started trying to make semi passive income with "mass mailing", instead of CPA I did SaaS affiliates, and chose to promote SaaS like Jasper.ai, but it seems like they hate the way we get traffic, even with a landing page as a buffer, they still terminated my affiliate account.
I figured I do not want to promote something, and get cut off when they keep the referrals, so I changed to promote my own services/products/dropservices with "mass mailing" instead.
The other thing difference is, the "mass mailing" part, I too have a huge B2B email database, but I figured the cost to send that massive emails, and have to deal with the inboxing issues, IP rotation, domain rotation, email warmup, I chose something which I believe is a simpler alternative, which is mail with contact form.
With more than 10 millions URL list with contact form, I can now send out millions of mail weekly without having to deal with inboxing issues, the only disadvantage with this is of course I will never be able to market to sites with a strong recaptcha (I'm aware of recaptcha breaking, it's just too slow for millions of sites), or sites straight up that doesn't use contact form.
I'm planning to also add mass mailing like you did but then I have 2 choices here, built my own SMTP server, or buy bunch of burner Gmail account, which I'm going to try next.