Neither closed or open software is safer than the other in my opinion. If someone wants to find a vulnerability they will find a vulnerability. The only advantage open source maybe has that it’s harder to hide vulnerabilities for years and it’s more obvious if they don’t fix it. But personally I wouldn’t use open source just for safety reasons.

Maybe check if your phone is stopping apps that are in the background. If I’m not mistaken it’s an Android feature and most of the time you can disable it in settings per app. On my Huawei phone it’s Settings > Apps > the app > power usage > launch settings.