Parking lot will be open when you get there. Don't worry about that.
Now this is what I would do, and it GREATLY depends if the park has everything up and running for ert. Things have been pretty dicey there so far this year on my 3 visits.
Anyway,
Enter at the marina gate for ert and go straight to Valravn. After doing that, walk back to Millennium, get that in. Take Frontier Trail back to Frontier Town and ride Maverick. Then walk over to the Gemini Midway to Dragster (Cubster) and wait for it to open. After riding that, hit Gatekeeper and Raptor. You may have slight waits for those two by then. Rougarou will probably be a walk on for an hour or two after opening, so you have some leeway there. After that, ride whatever you want...like my favorite...the CP & LE Railroad.
My best guess is that tomorrow May 22nd. The park will not be crowded. Have fun!!