This is my first VR game too!
Your solution is a thinking-out-of-box solution! But 70cm seems too far away (It did work fine for me, but what will happen if somebody with shorter arms try it?) . RigidBodies will kill perfomance if you hit fast enough.
But checking the average speed of the collision in some frames or ms seems appropiate to me, in this case you need to assure the game runs smoothly at 120fps all the time to get the most fiable avg speed.
Maybe the webXR export is faulty in some way? some other webxr games of this jam didn't recognice my controllers, I will be glad if you try my game in the webXR version to check how it works, I tried to optimize it a lot.
I had a little perfomance issues on the pcvr version, mostly of them at the start loading the shaders and materials. Specs: Rx 5700 CPU ryzen 5500, also I did a lot of little jabs at the start to check the fire