Garage upgrade from 20 AMP to 50 AMP
Buried conduit is considered wet location. Romex typically refers to NM cable, which isn't approved for wet location.  Depending on conduit size you might be able to run 6/3 UF if you can find in quad (not flat) form, but it could be very hard to pull.
Also 6/3 costs about $2.50/ft, you would be better running 3@ #6 THWN at 0.54/ft and #10 ground @ 0.40/ft. (Actual price at my local Lowes). It would be cheaper and much easier to pull. You would need at least 3/4&quot; conduit.
Protect the feeder with a 60A breaker in your main panel, and use a panel in the garage with a 60A or greater main disconnect, and ground rods. Buy a panel with many more spaces than you think you will ever need.
250 feet does have some voltage drop considerations, but the 40A load you describe will not create a problem even if full load at once.
Edit: I see you added it is 1/2&quot; pvc, you'll never get #6 to fit, the largest wire you could legally fit is #10 THWN protected at 30A. I also see you added it's only 60 feet, voltage drop should not be a problem.