Again, definitely tyres first.
Especially on the rear. Put a Maxxis Ikon or Crossmark on there, or possibly something like a Conti X King. Then keep something still reasonably knobbly and grippy, but slightly faster rolling on the front, like a 2.2″ Rubber Queen in Black Chilli compound, or a Maxxis Ardent or similar. That will make a huge difference to your speed!
Some lower rise bars may then (relatively cheaply) help you gain a slightly more weight forward, lower CoG position for the road riding and the climbing, but to be fair on most mountain bikes you’re sat quite upright anyway compared to a road bike.
Clipless pedals aren’t going to make the huge difference a lot of people think they do. They’re slightly more efficient over a distance, but you won’t notice any outright speed gains.
Rims can be a good place to save weight, but the EN521 is only about 70g per rim heavier than the 719’s you’re looking at. You’ll save more in the tyres for sure, especially if you converted to tubeless at the same time! Also, rim weight won’t really affect your speed, lighter rims can help you climb a bit easier, but won’t make any difference on the flat. I’d say to make any real difference you’d need to be looking at something like Stan’s Crests, or any other sub 400g tubeless compatible rims. Otherwise you’re really not going to notice it like you hope you might! And without knowing you as a rider, you may then be getting into the problem of wheels too flexy/flimsy for your riding style.