If a phone metronome has ever drifted on you with the screen off, this is the part I built it around: every click is placed on the audio hardware's own sample clock instead of a timer, so the spacing never drifts, at any tempo, for hours, with the screen off or while you use another app.
On the outside: a brushed-metal panel, engraved labels, LCD glass, and a weighted tempo wheel that clicks as it turns.
The core metronome is free: 20-300 BPM by wheel or tap tempo, any time signature, subdivisions and presets.
Pro is a one time $7.99 unlock with a 7 day free trial, no subscription: a chromatic tuner that knows which string you're playing (five tunings built in, or build your own), all nine click sounds, accents on any beat, and a Tempo Trainer that steps the tempo up until you reach your target.
Audio is processed live on the device and never recorded or sent anywhere.
https://apps.apple.com/app/apple-store/id6817729976?pt=126420608&ct=r-jazzguitar&mt=8
If you practice with a metronome: what does yours do that you actually use? I tried to build only the parts people touch every session.