I have a 21 ft. deck boat on a choppy lake with a muddy/sandy bottom.
Welcom Aboard. Av80r10146!
I have anchored too many boats to remember in every kind of holding ground there is for over 40 years of boating. I have used many types of anchors, but regrettably have never used a box anchor.
That said, my opinion and experience: In mud/ sand the best anchor is a traditional Danforth. Equip it with a boat length's worth of chain ( or as much as you can stand to handle/stow), use the appropriate scope, and it will catch, set and hold like nobody's business.
Now, teh only qualifier I;d add is that if you are anchoring over a grass/weed bed, teh danforth type is not ideal. The grass jams the flukes, preventing a good bite, and often requires multiple attempts to get a good set.
For this reason, I carry a plow anchor aboard as well. These work great in grass and weeds.