Please bear in mind that there are a lot of 3rd party apps on the Google Play Store that have not been updated by their developers for the changes in Android 11 in spite of having 8 months advance notice. Personally, I had to remove two 3rd party apps when I moved to Android 11 as they were causing overheating and battery drain.
If you have not already done so, try running your device in Safe Mode for a few hours and see if your issues persist. Safe Mode disables 3rd party apps and should help to narrow your issues down.
Another thing that you can safely do is to wipe your device's cache partition in Recovery Mode. This will clear out any temporary detritus left over from Android 10.
The 2 apps I deleted were a GPS location app and a live wallpaper app, IIRC.
The only way to locate which user apps or settings are causing an issue is to disable all 3rd party apps and then re-enable them one-by-one until the issue appears again.
The method to enact a cache partition wipe is in the link in my previous post to a video on how to do it, step-by-step.
Hey there DanTyler!
Sorry I took a while to look at your post, I've had a couple things going on this morning.
That's really weird behaviour for a week-old handset, so honestly, I'd contact the retailer where you brought the phone from and ask for a replacement. Repeated reboots can sometimes suggest a hardware problem, especially if they occur in quick succession. It could also suggest you may have further issues later on down the line.
If you do opt to replace your handset, make sure you back the handset up so you don't lose any files, and also make sure you remove your Google/Samsung accounts and perform a factory reset before you send the phone back.
Hope this helps,