not to be that guy, but isnt there a bunch of built in white noise/ rain noise/ etc built into ios accessibility

Yup, came here to say that too.

iOS Settings - Accessibility - Audio & Visual - Background Sounds

Offers the same as OP's app, except mixing. The only thing I miss is a a fade out possibility. For example, set timer at 01:00 minute and it slowly fades out over that whole minute. Much better than such an abrupt end. Not sure if OP's app does offer that.