With the use of a wood stove, I'd also look at using hot water. 50 gallons raised to 200F is probably 2x more heat storage than the phase change material.

Good point! If I have the payload for it I do want to work that in. What do you think about the feasibility of using a Vacuum Insulation Panel as the core in a SIP?

So, it also seems like you're not a fan of the phase change material in this application? It's not the best choice?

Mostly it's a matter of price and practical matters. Maybe build a small test box first.