I now use okmeter.io and really happy (especially for nginx and Postgres monitoring). They improve it constantly, and installation took just a couple of minutes. SMS/email/slack notifications work great (however, for slack, I needed to put a webhook).

:D I wish SMS wasn't so awkward, you're pretty much forced to have a credit system since it's so expensive. I'll probably still do it at some point. Makes it awkward for the customer as well if you have to babysit the credits

I suggested it before, but I think you can work around it: Let your customers give you their Twilio API key (with a big disclaimer that any charges by Twilio are not your responsibility...).

If I can get costs down elsewhere I'll maybe just do "unlimited" on some of the accounts, where "unlimited" is some large arbitrary number haha.

That would be the nicest user experience for your users, but it is a bit risky. You probably have a "reasonable number of notifications per user per month" in mind. As you sign up new users, you will sooner or later get some that will exceed that number by a lot--without a malicious intent.

You should just allow your users to provide their own Twilio API credentials and pass the costs to the user. You can even only make this option available on your plus and pro plans, increasing your ARPU[1].

[1] https://en.wikipedia.org/wiki/Average_revenue_per_user