Why does my iPhone fail to update via Mac Finder?

I have two iPhoneXRs (wife's and mine), two 2022 era ipads (wife's and mine) and a 24" iMac desktop (4 ports, 2023 model). I normally keep everything updated/upgraded promptly I have recently encountered catastrophic errors while executing ios updates on both of the iphones via finder from the imac, the latest occurrence today, 4/17/2025 updating ios 18.3.3 to 18.4.1 on wife's iphone. Both failure examples proceeded normally through download, extract, firmware update, software update, and verification to finder sign-off but then failed on the iphone restart attempt. I did the recovery mode restart and found myself presented with a message to the effect that the iphone cannot restart due to an unknown error. It was accompanied with Update and Restore options and a message to the effect that if Update cannot complete, Restore is required. I of course attempted the Update retry but it failed so I restored and updated successfully. Several hours of work and lots of nuisance tasks later I was pretty much back in business.

By the way the iMac needed a Sequoia 15.4.1 update today and I executed it before launching into the iPhone/iPad cleanup/backup/update tasks.

I used the iPhone XRs for over five years just relying on WiFi updates/icloud backups and for another year plus doing the housekeeping from the iMac. I have never had an update/upgrade failure just using home WiFi, icloud backups and doing everything from each device directly, and for the most part I never had any trouble doing everything from the iMac via Finder either. But these two recent catastrophic failures have left me somewhere between very concerned and terrified.

I am a multi-decade DOS/Windows denizen with a smattering of Apple thrown in. I would love some informed advice.

[Re-Titled by Moderator]