Some of the dead wood it getting soft and cracking pretty good. Should I let it do it's thing and rot away naturally or carve it soon?
So I spent a good part of the past week researching die grinders and bits and watching a lot of Graham Potter videos, thinking of designs and staring at the tree at all angles to see what can be done.Looking good, I'd let it rot naturally, unless you have a specific design in mind. If you have no plan, just let it rot naturally, let nature do the design.
Check out @MACH5 Eastern White Cedar Thuja deadwood Design for inspiration. The deadwood on the thuja had similar deadwood to your Dawn Redwood https://www.m5bonsaiworks.com/conifersChallenging tree for sure, what idea you have of styling?