If the company has full-time employees (ie, it’s big enough to pay salaries) and is already considering building it, the “I’ll save them money building it” value proposition doesn’t seem very compelling.

If you’re really committed to this, convince them you’ll run and grow it bigger than they would internally - probably by being able to raise outside capital for it with them as a significant shareholder (alongside the VCs and you and this PM as founders). Tie your compensation to success - which means equity, not salary (at least until you’ve raised new capital). Get the PM onboard.

I think the odds of that working are still low, but “You have a shot at owning 30% of something valued at, and exiting for, millions” is at least interesting and would be hard to execute internally.