Advice Needed: Simple and Reliable notifications

I have a long standing problem that probably has several good solutions, I just haven't been able to figure them out. So here I am, asking you.

I'm selfhosting some services, a mix of selfbuilt and open source software. But some things I don't want to selfhost. Notably backups and alerts/notifications. For backups I have a solution which works well in every regard except one - I don't always get alerted when things fail, because the way I send myself those alerts is failing more than the actual backups.

Currently I'm using python and gmails smtp interface to send myself email, but gmail disables my smtp access from time to time, and it's really easy not to notice not getting an email. I've tried sending the email regardless of whether the backup failed or not, but I've noticed several times that I still don't notice if the they stop coming.

Now on to my requirements/wishlist.

1. I'm already using s3 glacier at aws for the backups, so preferrably something in the aws space.
2. I would like to get an popup/toast on my phone when a message is being sent. And the ability to review messages later.
3. I would like as few moving parts as possible.
4. I don't want to write my own client.
5. I want it to be cheap, and if there's a cost I prefer to pay it at a place where I'm already paying, meaning aws (or possibly proton).
6. I want a stable service.
7. I prefer to manage as little as possible of the infrastructure.
8. I'd like a simple programmable interface that can't easily fail. E.g. http based.
9. It's no problem if messages are not received instantly, I could easily tolerate delays up to 24 hours.

As you may have noticed I'm pretty much expecting there to be something in aws that I can use, but aws documentation is so abstract, that I often don't understand what the point of something is or how I'm supposed to use it.