Follow Manager

Folders and slow, paced unfollows for your own Instagram following list. Runs inside instagram.com in Safari.

Install the script Open Instagram

Set up on iPhone

Install the free Userscripts app from the App Store and open it once to pick its scripts folder.
Open Settings › Safari › Extensions › Userscripts, turn it on, and allow it for instagram.com.
Come back to this page in Safari and tap Install the script. Tap the aA / extensions button, choose Userscripts, then Install.
Open instagram.com in Safari and log in the normal way.
Tap the FM button at the bottom right.

Good to know

No password is ever entered into Follow Manager. It uses the Instagram login already in your browser.
All requests go to instagram.com from your own phone. This page stores and receives nothing.
iPhone freezes Safari when the screen locks. The schedule only moves while the Instagram tab is open and on screen.
Automated unfollowing is against Instagram's terms and can lead to an action block. Keep the daily limit low.

Also works on desktop Chrome, Edge and Firefox with Tampermonkey or Violentmonkey.