Advice for May entry

I couldn't find an answer on any other thread; apologies if one exists.

I am planning a trip to the park May 18 (Sunday), May 19 (Mon) and May 20 (Tues), staying at a hotel outside the park boundary.

My plan was to buy an entry permit in my rental vehicle at a park gate. This seems to be the only viable way to get entry in a vehicle (I have booked a YARTS ticket for only one day, as a backup).

Is this a reasonable plan, if I show up very, very early each day?