Increase in velocity isn't what you're looking for in a longer barrel. Theory says a longer barrel will slow the bullet some, therefore stabilizing the velocity........giving smaller Extreme Spreads in velocity.
16 to 21 inch barrels will give similar velocities, at about 22 inches velocity...
This is why the CZ 457 is a good start. You can add to it as you go or leave it like it is..........and have fun while doing it!!!
The Arken scope you have will be a good start.
CZ 457 Varmint
CZ 457 Pro-Varmint
CZ 457 At-One
CZ 457 Varmint MTR
On a budget, the CZ 457 is a very good place to start. You can upgrade the rifle as you go. There are several good options for chassis'. Lilja makes barrels that will definitely up the accuracy level, if needed. Also several aftermarket triggers available if you can't work on the factory trigger.
This is the one I like...Ruger 77/22 hornet stainless varmint, 24" barrel. It shoots well under MOA.....been quite a few years since I've shot it. The only rifle I own that I've not handloaded for, I have everything, just never got around to it.
Hope you find one to try.
Always good to meet shooters that are willing to help!!
If and when you get ready for wind flags.......Graham Wind Flags are the absolute best that I've tried.
https://www.brflags.com/
Here is a good example of how the wind effects my shooting. These are 10 shot groups at 50 yds, same ammo, same rifle, same day, on an ARA Unlimited Target.
First pic shows a group in steady 6 to 8 mph wind......second pic shows a group in a wind that was change a lot 2 to 15 mph. Both winds...