I use rolled soy flakes. I think they are pretty much perfect for this purpose, but unfortunately not so easy to source

How much protein do you actually need though? If you're not using it immediately (i.e. shortly after exercise) it's wasted.

With just the oats it's hard. What I do is ferment my own greek yogurt (milk and starter in an instant pot for 9h, then strain it in the fridge overnight) and eat that with müsli mixed in (the German kind that's nothing but whole grains and some raisins, not the garbage that's basically breakfast cereal). Tastes great and gives you a ton of slow-release energy and protein.

The study is suggesting two days of intense oats. You can go totally without protein for two days and barely notice it as long as you're keeping yourself full, and a big pile of oats does a surprisingly good job of that.