Thank you VERY MUCH Mustang7757!
I was too worried that something could go wrong and I could no longer return the phone, so I moved all my stuff and replicated all settings in another s10+ and then went for a factory reset of the previous one.
In case it's useful, after the Factory reset the Android update seemed to complete correctly.
I noticed that the phone showed "Sprint" in some messages. Wondering if it were originally a Sprint branded model, then turned into a T-Mobile one, and if that's why there were issues during the update.
Since I have everything in order on the second s10+, I'm keeping that one and returning the first.
But I am also keeping your instructions, and will try them on an old Samsung, so I learn something new, without risking too much.
Again, THANK YOU VERY MUCH!