iMac will not update to newer software

I have a 27" iMac, purchased in July 2019 (3.6 ghz 8-core intel core 19;3 TB Fusion Drive;32 GB Memory).

Two months ago, for all intents and purposes, it went belly-up. It would just spin and not get past the user sign-on. The local Geek Squad ended up sending it away to be fixed (it was and still is covered by Apple Care). They returned it with Mohave installed (it went out from me with Big Sur).

Ever since getting it back, I am unable to complete any newer software update. I've tried Catalina, Big Sur, and Monterey. They always end with an error, usually "an error occurred..." or "the network connection was lost." I have just updated my MacBook Pro with Monterey with no issues. Our network is working. The iMac has tons of memory/space. I've been trying for over a month, several times a day, and night.

Any ideas what could be wrong?

P.S.and for the record: I was NOT impressed with my first and last dealing with our local Geek Squad. They were the only Apple Authorized Repair place within three hours of where I live, and I utilized them due to my Apple Care protection. They had my iMac once and returned it, still a brick, and then a week later, kept it for over a month, and returned it like this.