Gusto's 2o2o is pretty rock solid - I was super proud of how well I'd trained it, although it is turning into a major pain as I'm currently trying to break it and he is convinced I'm just proofing him!
I wouldn't be using my hand target to get him off the board; it makes the focus (for him) your hand and not his feet. You want him to think about what he's doing with his feet, not think about reaching to target your hand. If he doesn't already have the skill, I'd be doing a lot of rear-end awareness exercises away from anything resembling equipment. Teach him to offer swinging his back feet up (or backing up on to) things like books and boxes and pillows. Make sure he understands that he can move his back feet and that moving them on to things brings rewards. I'd then move it to a board; not running the length, but just backing/swinging on to it like you did with the other objects. If you are going to use a verbal cue for the contact, that's when I'd introduce it.
Once that's easy, I'd hold him by his collar/chest just foot or so before the end of the plank. Get him amped up a bit and pulling on you, then release him as you give your verbal cue for the contact (or not, if you aren't using a verbal cue). If he moves into the position, jackpot the heck out of him, give a release cue, and have a party. If he doesn't get it after a try or two, go back to working just the end of the plank and get that stronger.
Eventually work up to adding more distance on the plank, more duration to the time you ask him to hold it.
I know there's a million roads to achieving a nice 2o2o; that's what worked for me with Gusto.