OK first ... loose the harness. You can't correct a dog with a harness, engineers notwithstanding harness are designed for a dog to pull. And you actually caught a break with the "Dog Park" thing ...not a lot of "Dog Park" fans here or as I am want to say "No one in there right mind would subject there dogs to the uncontrolled Chaos of a Dog Park." Of course there are exceptions but it's a good rule of thumb.
The Crate is about more than keeping the dog from tearing stuff in the house. It's about setting rules and boundaries. The other dogs thing .. well I don't know what your plan is there?? But I advise people not to screw around with "Is your dog friendly people??" By and large most folks have no idea and those who do .... won't do that. I keep my distance you keep yours ... my dogs are taught to ignore other dogs, works out fine. But you can't get "there" until your dog knows how to go on a proper structured walk., accomplishing that ... is job one!
And it's done in a distraction free environment using a proper tool. A flat leash and regular collar a Slip Lead Leash or a Prong Collar. And a correction is a slight tug sideways or ... a wrist flick if you use a "Prong Collar. My tool of choice is a "SLL" but I now realize using that tool correctly is more of an "Art Form" than a science???
So halfway thru the thread I started on the SLL I switched tactics and it just changed to more of a "concepts of walking your dog thread. There is some info on a Prong Collar but the "trainers" who have Youtube clips with lots of info on "Prong Collars are there ...ie Tylor Muto, Jeff Gellman and Sean O'shea. So you can look those guys there for additional info and ask here of course.
Slip Lead leash - Boxer Forum : Boxer Breed Dog Forums
As to the jumping the best approach is to Correct the dog for jumping! I use ... the "Knee" thing myself, dogs don't jump on me. But yet again more of an Art Form than a science. I tried to show my rather ...awkward friend how to do ... that with my "Boxer" Let's say that was quite humiliating for hi but entering for me to watch.
But lesson learned the "knee" thing can stop a dog from jumping on you but ... it's not an easily transferable! IE the dog says well I won't jump on that guy ... but everybody is raw meat!
The "Sit" first thing ... may or may not work?? It's only effective if your dog has a clue as it were. If you have to do the Sit Sit Sit thing?? Your wasting your time. I have had hands on first time experiences with dogs I have never seen using a SLL. And dogs offered a "Sit" as a behaviour in less than one or two minutes. I never said a word ... so it was nothing I did. So I don't see much value to "Sit" with a dog that has no manners?? In addition if the dog is poorly mannered in the first place if he is in a Sit ... well he's already halfway to his target.
If you do/did use a "Prong Collar" you'd just keep a leash on the dog (let him drag it) and when he goes to jump ... step on the leash and stop him .. he'll get the message pretty quickly ... jumping is not allowed! Or teach a proper down and stay! Once taught ... "Down" is your correction ... that's is how I taught Struddell to stop jumping on uh ... "everybody else!" She kinda sorta hated down becasue it meant ... "PlayTime is over!" After a message received was accomplished with a few "Downs" at first signs of "trouble" then ... Sit was fine.
Or you could just use a "Pet Convincer" and call it a day.
You can pick one up at a local Bicycle Shop for half the cost its a Bicycle Air pump.