You have picked the right size anchor off the Rocna guidance. Remember they use metric tons (MT) of 1000 kg. A MT is about the same as a long ton (2240#). You have lots of anchor capacity between your boat and the 10m, 8 MT limit on the Rocna 15.
I've been sailing with a Rocna 25 on my 12m, 10 MT boat for two years. In Chesapeake mud I always bring up a good amount of the bottom. I think what you have to deal with depends on the bottoms in your normal cruising grounds. In sand it is now big deal. In the glue we call mud around here it takes a deck wash and (sometimes) a boat hook to clear it.
In my opinion all that mess is an indication of holding power. I'm happy with my choice of the Rocna and wouldn't hesitate to make the same choice again.
sail fast, dave