Email is expensive on purpose to incent customers into following good practices. Otherwise you attract all the worst customers (“let’s email everybody who’s ever interacted with us for a decade, regardless of if they want our emails!”). Then Gmail starts putting all emails sent on the cheap platforms IPs to spam by default.
Based on your practices, it sounds like these pricing models are doing their job of correctly scaring you away.
Gmail's algorithm has really tightened up this year. Open your spam folder right now and you'll probably see more than a few legit businesses in there.
Basically if your engagement (opens, clicks, unsubscribes, complaints, etc as a percentage of total sends) is poor enough, Gmail will dump you in spam by default. They don't just "trust" that you have permission to email the people you're emailing. So many businesses abuse this, and they're incentivized to keep your inbox clean so you keep visiting your inbox (and click those Gmail ads!).
If you're getting low/declining engagement over time the algo just assumes you have poor sending practices, which to be honest, if that's happening, you probably do.
So the word deliverability includes making it past spam filters?
Then your domain reputation will be good because Gmail/outlook’s algo will see good numbers coming from your domain. Then the only thing you have to worry about is your IP rep (which comes from the platform you’re using — basically “are you sending from a platform that has high standards for customers and zealously guards their IPs”).
You can spend $thousands on consultants and what they’ll tell you will basically just boil down to that.