I have iStat Menus on my Mac showing me my SSD usage (R/W) in my menubar at all times. I have rarely ever seen it go over 1GB/s, and I'm on an M1 Pro MacBook Pro, using it pretty heavily. I see 1GB/s when using the xip command to decompress Xcode bundles. When doing gatekeeper checks on large app bundles, I typically see somewhere around 500MB/s, bottlenecked by the CPU performing the checks.
What kind of work are you doing where 1.2GB/s isn't enough?
Paging. Part of something not mentioned was also IOPS. Are you on 8GB with your M1?
With 8GB you will have lots of swaps happening. Especially with Safari. That extra SSD is going to make quite a lot of difference. Together with 12GB Memory this would be enough for ~90%+ usage.
What kind of work are you doing where 1.2GB/s isn't enough?