I had the S21 Ultra a couple years ago (a $1,200) phone at the time, traded it in for a phone half its cost (pixel 6) because I found after owning my first Samsung that I did not care for their practices. This beast of a phone had issues constantly stemming from the heavy UI skin over android to the spilt in two ecosystem of Google and Samsung. Pre-installed forced upon software as well such as Mcafee didn't sit well with me either. All that at the time "state of the art" hardware bogged down by all that ridiculous nonsense.
That said the only solution I found to security that I felt was necessary and helpful was to sideload a VPN with excellent ad blocking and download protection. Something along the lines of Nord or PIA. Im not a fan of placing heavily restricted antivirus software on mobile now days. I see no need for it when you can find the protections one would want and need in a VPN and have full system encryption as a bonus. ALways download apps from either the playstore and or legit websites. Watch what you divulge and always check those links/URL's. Password protection can be obtained through an app as well with either open source applications or through companies with your VPN like Nord. I wish you luck with that device, they do have nice displays, excellent camera systems, ect, its just a shame Samsung feels the need to load them down the way they do.