It is always nice to see a fellow Trekship fan who is concerned about contradicting other fans.
I'm really loving the spoiler, which gives her a very sporty appearance.
It would seem you still can't quite do proper phaser strips with the rounded ends, but who am I to judge, especially as I can't even model in 3D at all? Besides, I know full well that she is in her early stages.
Interestingly enough, I looked up Mitchell in my logs, and there was a Ptolemy class vessel, the U.S.S. Mitchell, NCC-3863 (I believe this designation was invented for a Jaynz sourcebook), not that it really matters in this case, as the Ptolemy class vessel of that name would have long since been decommissioned. Just an interesting footnote.
I would suggest avoiding any lettered registries if that is what you are planning (like NCC-1495-A), so as to avoid contradicting the aforementioned Ptolemy class Mitchell, unless she is a much earlier vessel that was somehow lost, and the Ptolemy class vessel then took on the name.
If she is a Defiant class, it would be better for her to have a registry higher than 72405, and just be done with it on the safe side.
And that brings me to my final quip on the subject: she really doesn't even look like a variant of the Defiant class, any more than the Avenger, Valiant, or Vigilant classes do, which look, and in fact are, more like offshoots of the Defiant class. They use the Defiant class as a basis for their design, but were constructed from the ground up to be improvements upon the Escort Cruiser/Corvette.
Maybe it is because the model is in it's early stages, but so far, her components only look vaguely like those that are also used by a Defiant class, and the Mitchell looks unique compared to the Defiant class.
I hope you get what I'm saying without coming off too negative.