There are great non-Samsung options available right now for $400-$500, brand new, with current technology. They all run Android, so they'd be familiar to you coming from the S3, though not identical because they would lack Samsung's goofy overlay.
The Pixel 3a or 3a XL is the obvious no-brainer choice to me ($399, $479 I believe.) Google makes Android, and Google makes that phone. It's inexpensive, runs great, and will get security updates for 3 years. The S5 you bought is already dead in the water on updates.
For a little more money and a LOT more power, the Asus Zenphone 6 is a steal at $500. If I was buying a phone today, it'd be that one. Current model, cutting edge processor, big battery, happy times.
Return the S5. If it wasn't free, you overpaid.