How long can I let my Yaris sit before risking damage?

[RMcG]

I have a 2008 Yaris hatchback that I currently drive pretty regularly. But I own another car (a Hyundai Elantra) that is driven primarily by a relative. Because of the pandemic, the relative is not here to drive the Elantra and it has been sitting for months, although every 4-6 weeks I go out and start it up to charge the battery and let it idle for a half hour. And to prevent flat spots on the tires, I pull it out of its parking space and go back and forth over about 30 feet before re-parking it.

Because of the pandemic, the relative who drives the Elantra will not be here for another 6 months. So I have concluded that it would be good for the Elantra's maintenance to reactivate the liability insurance and actually drive the Elantra for a while. So I want to deactivate my Yaris's liabilty insurance (to save money) for a while, drive the Elantra, and do the same "maintenance" drill with the Yaris as I have done with the Elantra. That is, go out and start my Yaris up, recharge the battery, let it idle for 30 minutes or so, and pull it out of its parking space and move it back and forth to prevent tire flat spots, maybe every 2 weeks or so. My Yaris is my primary vehicle, so I want to be sure I do not take too much risk of damaging it.

QUESTION(S):How long can I do this with the Yaris, before I risk damage to the Yaris? One month, 1.5 months, 2 months? Is synthetic oil better and less likely to degrade with less driving & use? Does anybody have any other tips or suggestions?

Thank you,

R.