I have a Garmin Edge 1040 which is awesome and I use a Cospoo HRM with it which has outlasted any Garmin HRM I’ve ever owned so I wouldn’t be concerned about using their bike computer if you want cheap & functional.

Not sure why you need GPS if you’re not navigating, surely you’ll just need a bluetooth speed sensor and hrm connection? Assume you’re not using a power meter given your comments above.

Whether there is a low cost device that doesn’t do GPS but does those other things with a decent screen, I’m not sure

Yeah that Cospoo or the Garmin 130 should do the trick.