Shimoji district and Shimojishima are different places. Shimoji district lies in southwestern Miyakojima and includes the area around Yonaha Maehama and the route towards Kurima; it is not the island with Shimojishima Airport.
Rental-car pickup and return points differ by company. Match the airport names on your flight ticket and rental booking, and leave time between returning the car and check-in for early departures.
Miyako Airport · central Miyakojima
The airport is in central Miyakojima, southeast of central Hirara. Accommodation in Ueno, Shimoji district, Gusukube and other areas on Miyakojima itself can be reached without crossing a bridge.
Choose the route according to whether you will go straight to your accommodation or stop in town. On the timetable for your travel day, look for a bus serving the stop nearest your accommodation.
Shimojishima Airport · Shimojishima, next to Irabu
The airport is on Shimojishima beside Irabu. To reach central Hirara or accommodation on Miyakojima itself, travel through Irabu and across Irabu Bridge.
Travel requirements differ between staying on Irabu–Shimojishima and staying in southern or eastern Miyakojima. Choose an airport bus whose route serves a stop near your accommodation and whose timetable matches your flight.