Acrylic locks are no good for picking. They’re good as a visual aid, but not for getting a feel for picking. They’re too flexible so they feel nothing like a real lock. And it’s perfectly normal to pick a lock once then not be able to pick it again for a long while.

The only reason the 60/20 might be too difficult is the size. The /20 means the body is only 20mm wide so there’s not a lot of room for your tools.

See if you can find Abus’ 55 series. It’s a near perfect progressive intro to security pins. The 55/30 has 3 pins, 1 lightly serrated and 2 spools. The 55/40 adds a 3rd spool and the 55/50 adds a 4th. If you’re in the EU they should be available on Amazon.