I just bought my standard S21 this afternoon and have spent half the afternoon trying to work out how to fix this exact problem as well. Finding it quite frustrating for several reasons. Sure I can mute system sounds altogether when I don't want to be blasted with simple in-app 'system sounds' like sending / receiving a new message on messenger, but I'd rather have them there like I'm used to but at a normal volume. Having every single volume option sitting 1mm off of muted still results in loud sounds when apps are open. This is definitely going to bug me if not fixed at some point.
I have the same issue with my Galaxy S21 Ultra with the last firmware update with march 1 security correctives.
It's really system sound who seems maxed out whatever the system volume you set, it's like max or off.