I hope the other commenter's idea of "right on-call practices" involve hiring enough employees to give a humane on-call schedule, and compensating appropriately for the on-call time. That's my idea of the right on-call practices, and would go very well with "add 2 zeroes" in the pricing of that SLA.

But, yeah, a lot of places don't do that.

I had a manager tell me we can't hire people to be low productivity people just because we need people in the rotation. Idk if I agree with that, though I guess I get the point.

Who suggested hiring low productivity people?

If there's not enough work to go around but you need people on the rotation your productivity will be low

In that case, just contract with skilled infrastructure consultants (whether as individuals or through multi-person consultancies) to share on-call duties with your infrastructure employee workforce on an as-paged basis rather than hiring them full-time.