I cofounded OneSignal for this reason. We were previously a game studio that built an in-house solution. It was a huge maintenance headache and a distraction. Eventually, we realized that so many companies were one-off push implementations and decided to focus our efforts on building a really great third-party solution.

Firebase and SNS have since entered this market as well. I'm obviously biased, but I honestly can't recommend them. They work for some of the basics, but there's been very little innovation in either product and occasional service issues that go unsolved for a long time. They also don't make any money from them, which should be a concern for anyone that plans to use these services for such a core part of their business.