If you are stuck on 14.7.1 do you have a beta profile in Settings, General, VPN & Device Management? If yes then remove it and allow the phone to update.
Try Settings, Health, Data access & Devices, Tap on your phone in Devices, Tap on Delete all data from "your phone".
If you cannot delete the data in the Health App then, because the Health App itself cannot be deleted, you could
Backup, restore to factory, and restore from backup
This will update your iOS to 15.4.1 if you removed any beta profile.
If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails.
(1) How to back up your iPhone, iPad, and iPod touch - Apple Support
(2) Restore your iPhone, iPad, or iPod to factory settings - Apple Support
(3) Restore your iPhone, iPad, or iPod touch from a backup - Apple Support