You could try initializing the Screen’s.BackPress() event to get the ad to close, but, other than that - I don’t think it can be done. Google has reasons for everything and it’s good practice not to try and circumvent the built-in blocks to try and do something with their ads, that they don’t want you to do.
If you’re getting complaints from your users that Interstitial ads are too invasive, you might consider using only 1 banner ad. (Also, you never want to overload your app with ads. Users don’t like that, and they’re likely to uninstall and downvote your app)
Hope that helps