If I'm early with the extension, I'm pushed into the back seat and I have to make a recovery. If I'm late, the tips don't engage and hook up so "plop" I'm sitting on the snow.
When I get it right. WOW! Cool feeling..
It took me a while to get the difference between extending the leg and extension from the ankle. Prior to working with Mac Jackson last year, I just never had any conscious control over ankle movement in my skiing. From your description, I suspect you are not getting it yet.
Mac started my journey by having us do little hops on traverses to discover the difference between leg extension and ankle extension. Leg extension hops invariably involve a preceding down move and generally get a higher hop. Ankle extension hops by definition can only go as high as the ankle extension and don't require a down move. Throughout last season Mac had me doing various exercises that built on this ankle movement.
You can do some experimenting at home. Find a spot facing parallel to a wall and stand arms length away. Stand up with your legs straight and extend (open) one ankle. This should result in a pure lateral move (shifting weight to the other foot) and tipping of the shoulders. Try the move again with your knees bent over your toes. This should be a little harder (less balance), but end up with the same result. From the knees bent position, now try bending your inside knee after you've extended from the ankle (use the wall for support if you must, but you should not need to). You should feel the weight shift to the balls of the feet and feel the hips come forward. Now if you combine the ankle extension and the knee bend into one simultaneous move, you should get a sensation of a need to push your inside hip forward and a definite need to use the wall for support. This is getting close to what we want in our skiing.
Now try the same set of moves again but starting from a countered position (upper body parallel to the wall, but feet turned 15-20 degrees away from the wall). Just extending the ankle will strangely put you in the back seat over your inside foot. Doing the simultaneous move should feel much more natural with the upper body squarely going fore-agonally into the new turn and the feet tipping onto the new edges. This should explain why the ankle extension should not be a too early timing issue for you.
When you're late, you've already done something else to do initiate your turn. It's the up move. That gets the skis flat. That allows twisting and tipping the skis. Finally I see inside leg flexion to collapse the COM to the inside of the turn just above the fall line. Look at the left turn from 4-5 seconds (with the gondola going by in the background). Do you see that the initiation extension is up versus having an "in" component? Can you see how flat the skis are in the snow just before the fall line? Can you see that the maximum horizontal distance of the upper body away from vertically over the skis occurs after the fall line? So it's not that you're late, it's that you are not doing it at all. Since we don't see you "plop" in the clip, I guess there still could be a too late timing issue. But I doubt it.