I realize they’ve become a behemoth in the space, but I may have missed what made them “bad”.

If you find a decent alternative let me know. I have been looking for a while and not found anything that supports the full feature set I want (including Twilio).

Big public clouds will provide SMS send as service e.g. Azure Communication Services or AWS SNS. You can uses these services without hosting your workload in those clouds.

Are you trying to do something programmatic with thousands of calls, or just engineer your own bespoke stuff? Seconding AWS SNS for big things. For small projects things, Google Voice at least used to be free or cheap and let you treat phone communications much more like emails than a standard phone number does.

I've used plivo for a project I had built previously for a client. Pretty straight forward API