The most secure way to store bitcoin is offline in a hardware wallet or deep cold storage if you are not planning on touching it anytime soon. Anything touching the internet has a significantly larger attack surface because remote attacks are much less expensive to carry out than physical attacks. That being said, a mobile app is more secure than a desktop app because mobile phones come with lots of security features enabled by default that desktops don't have or require to manually enable.

If you're not ready for the jump to a hardware wallet, would still recommend you look into ways to generate your private keys from physical entropy rather than using the pseudorandom number generator (PRNG) from the Coinbase app -- not your keys, not your crypto. Type "using dice to generate keys" into Google and you can find some steps to get you there.