20 hours per day confinement or 20 hours total for the week?
Either way I personally don't feel you're failing at all. Training classes (check) Socialization and exercise at the dog park (check) Walking/potty breaks with a dog walker (check). I don't see anything that a GSD needs missing.
As far as crating IMO it depends on the dog whether it can be given free roam. My Zeeva didn't get roam till about 3 1/2 years old. Some dogs actually feel more comfortable (and are safer) in a crate so don't feel guilty about crating a GSD.
I'm curious though; what do you feel is so different about sighthounds when compared to GSDs?
I must also assure you from my experience that 4 months is really young. It gets so much easier after about 2 years TRUST me. I cried a lot after I got Zeeva because I felt like I was failing her and wanted to rehome. But she's turned into a fine young lady now and I'm happy I didn't find her a 'better' home.
As far as the bonding, it took us a while to develop that so don't let that bring you down...
Hang in there. I assure you, you sound like you're doing well. The only thing that I feel might be missing between your pup and you is a bond (and honestly that may not develop)...but be patient for at least a year? Then reassess...
That's my experience and opinion...