I had the same issue installing a new faucet in my parent's kitchen, old faucet was 1/2" IPS, new faucet expected to be connected to a 3/8" flare male fitting. I cut the PEX and used a pair of sharkbite to 3/8" flare quarter turn shutoff valves to make the final connection. Easy, and now there are reliable turn-off valves in place.
You don't mention if the faucet input is rigid or flex lines, or whether the unit side is PEX all the way to the 1/2" or if there's flex there and something different further upstream. Those details might suggest other options.