how fast can an iPhone be erased?

tech repair at a local small town repair shop, says it then needs to download and install software... took phone to the back said it'd be done in 30-40 minutes. phone was on iPhone unavailable lock so asked him to restore to factory settings. Just want to reset to factory to default so I can restore from backup. want to make sure he's not using software to access data on the phone.

phone was locked out " iPhone unavailable", without erase button on the screen.

Apple Support Rep said only option was to hook up to a Mac or Laptop, go to device in Find, and then restore to factory settings.

Repair guy said after it's restored, phone will then need to download and install updated operating system & it takes 30-35 minutes usually. Again, needing to make sure he can't hack into the data in the phone.

I tried last week to erase via Findmy iCloud login for lost device, but it never went back to the hello screen. "iPhone unavailable" stayed on the screen, even though it was still charged and in same wifi. It was originally on "Security Lock" after my dad had too many failed code entries. Elderly dad with shaky hands and shaky memory kept reentering wrong Home Screen passcode for 2 days. When I hooked it up to iTunes on my MacBook, it ran updates, 8 hrs unavailable message, and when I entered code, it eventually popped up an "iPhone Unavailable" screen. I'm not there to troubleshoot, so took to Verizon to reset, but they didn't have a Mac to plug it into. That's how he ended up in a small ifixit it strip center place. Authorized Apple repair is an hour away, too far for him to get to.