Major help needed iMove iMac Audio Sync issues

[guitarman1996]

DETAILS:
Doing videos for a guitar pedal videos, I play guitar, talk a little, and show how the pedal works... Simple enough, thousands do it!

Started using my iphone 11 in 4k 24 or 30 fps, and running a mic in front my guitar amp and into Grageband to create the audio.... Simple enough, millon's do it!

PROBLEM:
The audio keeps going out of sync as the video plays.... Never had this issue until I started using 4k. I start by using the clap method to get sync and it works great, but after a minute the video gets behind the audio and it gets worse as the video plays on.

Please help! It's driving me nuts!

 

[mirrorlessNY(youtuber)]

In 1080p everything is fine? keep doing 1080p until you figure it out,, meantime can try using a 2nd cellphone or some audio recorder to record audio separately, can also try a different phone to do same procedure.