Good points. I do think Ohtani will break any previous scale for mid-season rentals though for the exact reasons you outlined. Any team that gets him is getting impact starter and bat, and assuming they are in contention probably have a leg up on signing him.
Knowing this, if the Cubs are serious about Ohtani, it would really help them to be in contention at the deadline. If the Cubs are under .500 they probably arent trading for him and a Dodgers/Mariners (due to west coast)/Yankees/Mets team has 2 months to make an impression on him. He'll still probably hit the market and will still probably go to the highest bidder, but it may allow him to be more open to exploring playing on the east coast or elsewhere.
Feels like the Cubs best chance for Ohtani is trading for him at the deadline and then offering him enough money to be in the range of other teams, assuming his experience here is positive. Otherwise, they might have to actually be the undeniable highest bidder to have a shot.