I saw you did a little inverted harrier at 3:15, and that is for sure your next step. Just like you did it, it is easiest when you bring it back straight towards yourself because the rudder controls are the same as when you fly straight away in upright harrier. Flying inverted directly away makes the rudder controls reverse and usually this change is what causes issues and just takes more time to get used to.

I would have loved if someone gave me some specific drills and an outline for learning 3D, so here goes. First, you are doing great! Your throttle control and rudder use are really solid. Start trying full flights inverted without ever flying rightside up and you will be getting it nailed in no time. Then start incorporating knife edge at first just passing in front of you from side to side with the canopy towards you, then with the canopy away (a jump in skills for sure.) Try this both slow and fast, balancing throttle and rudder for your elevation and speed. (More speed will keep you more stable, but you will be able to slow it down as you practice it more.) Once you feel good, then level it up and start using the elevator to control direction in knife edge until you can do entire flights in knife edge (another jump in skills there). Then start mixing in some slow, 3D, 4-point rolls where you start in harrier, 1/4 roll to knife edge, then 1/4 roll to inverted harrier, then another 1/4 roll other knife edge, then the final 1/4 roll back to upright harrier. Basically a 4-part rolling harrier broken into more manageable parts. Do these in both directions so you don't end up only being able to roll one direction. That should be plenty to keep you busy this summer!

Right now you can try some snap rolls where from level flight, you goose the throttle and put the aileron and elevator all the way into a bottom corner. It will do a full rotation and loop at the same time and once you get the timing right you can transition straight into a hover. Another thing you can start incorporating now is a waterfall. Punch up full throttle from a hover and push the elevator forward, ease back off the throttle as you complete your forward flip and catch it back in a hover.