Get a used Android phone. (Craiglist, eBay, or ask literally anyone you know who might just give you a 3 year old phone they have sitting in a drawer) Just make sure it has working GPS.

I understand you don't want to ride with your regular phone mounted to the handlebars. I wouldn't do that either as 1 mistake and you lose an expensive phone. But this way lets you repurpose any old phone as a bike computer. It's cheap. You don't need to pay for cell service (as long as you have Wifi at home to install the apps the first time)

A 3-4 year old smartphone has MORE than enough computing power for this purpose. (GPS tracking and speed calculations are not heavy users of system resources. So even an old outdated phone will work fine for this)

The apps you need are free. I like Urban Biker. It does a full dashboard. Speed, distance, etc. But there are 20 different cycling dashboard apps.

And of course you'll need a mount for it. But those can be found anywhere for $10.

You can also put offline maps on it so you can see where you are without a cell connection. Or load music on it. Or anything else you can think of.

It's a GREAT cheap solution that's better than any store bought bike computer in my opinion since it's customizable to do exactly what you want it to do.