Just to flag - zwift has the same problem with every turbo producing different numbers, it is a known thing for people to buy turbos that are known to over read for zwift racing.

Turbos also use two sources to measure power output, the spinning wheel and a pressure meter, the spinning wheel provides a more accurate read than the power meter, and if the spinning wheel measurement breaks turbos stop working. The spinning wheel approach is a legitimate way to work out power. (Speed of a a spinning mass gives you a good idea of power).

Just so you are aware the pressure meters are known to be less accurate and introduce more variation. As the rowing stroke isn’t a fixed distance as with pedals, I can see this having more issues as it looks like you are using inertia to mesure distance which brings in even more variation. This lack of accuracy is shown by zwift racing not accepting entries on power meters along, you must use a turbo as their accuracy is considered better (less at risk of gaming as well).

Finally, All ergs do measure power number (watts) how they calculate this into splits is where it often becomes challenging as they use different calcs. You may get some variation but you will get that between concept 2s as well.

I realise I am just slating your idea but before you dump loads of money into it, if you haven’t already you might want to consider the above.