he other side of the valve, which is a female thread, looks to be about 3/4" inner diameter.
Buy a 1/2 inch nipple, and take it home for size, or, better yet, take your valve to the store, and try for size. Start with 1/2 inch nominal.
Given the parts you have, you may end up having to drill that hole in the escutcheon bigger. Also, you could get a new escutcheon with the right size hole. Ones made for a shower arm may be the right size. The new escutcheon could be shallow I think.
So whatever nipple fits the valve bottom right, my current idea is to adapt the PEX to a brass nipple. Adapter from PEX to nipple is below the floor. Escutcheon drops over the nipple before attaching the valve. Valve fits on nipple, and is positioned tight against the top of the escutcheon. Then install your long 1/2 OD tubing up to the faucet. I am not a pro, and I have not done what I envision.