First off, slam that seat and tip it back more in line with the top tube. You're on the right track with the drivetrain. I assume you're getting three piece BMX cranks for stremph. Twenty-six inch wheels are the way to go so punish those and build a nice set when you get better. Likewise, wear out those tires and get faster rolling ones when you need them. The fork should serve, though it would be nice if it was shortened, or at least got heavier oil. I can't see if it's an air fork but if it is higher pressure will firm it up. The only other issue I see is that stem. I've never heard of one slipping but I wouldn't personally trust it for dirt jumping.
I know this sounds bad but just save up a little and buy a dedicated dirt jumper off pinkbike. It'll hold up better, be a ton more fun, and you'll probably save a bunch of money in the long run.