In the SW lower peninsula of Michigan, I have township water and sewer, natural gas since the early 1950’s and all the calcium chloride a person could want for traction in the winter. According to the Holland Township rules, I may call for permission for an outdoor bonfire with no trash and a garden hose ready for an emergency for a fire footprint of no more than a 4 by 4 foot area. I receive permission 1/4 of the time. Last one was 2 months ago in which I had a brush pile approximately 10 foot wide, 30 foot long, and 6 foot high. The call verified the footprint rule which I replied that I would partition into said parameters. Instead, I lined up end to end two cars and a truck to block the fire view from the street and utilized the diesel sprinkle with a gasoline tail. An hour later it was done. Got rid of a Lazy Boy a week ago that way. And all this dancing with wolves only cost me $1300/year for 1 1/4 acres.
I heat my domestic hot water with wood in Grand Traverse County in Michigan. We have teenage children and they help with cutting the wood and firing. The unit is located in a detached pole barn, with water pipes insulated and buried well below the frost line. I made the heater myself from a 40 gallon gas water heater by building a firebox with refractory cement and firebrick on the bottom of the heater. It has served us nicely for the last 4 years, but has meant welding up a new set of grates every 10-12 months. Insurance wise it’s not a problem because it’s not in a living space, it’s an “outdoor” stove for practical purposes. https://www.youtube.com/watch?v=RDdTYFw_evY
I like it! I see you have an un-insulated pole barn which means you have to keep fire in the heater day and night 24-7. That will teach those teen responsibility. 
Do you have a backup in the house or is this it for hot water needs? Did your TP valve ever open if it got too hot? How many times a day do you have to fill with wood?
Regarding being outdoors, it actually does quite well about not freezing up in the winter. Only when it gets to -20F do the lines freeze, it only happened once or twice and then I put heat tapes on. The tank itself is nothing to worry. We typically only fire morning and evening, and laundry day every 40 minutes till about noon. 
We have an electric backup in the house that never gets used. The TP valve opens only if the teens aren’t paying attention and over fire it (can’t recall how long ago the last time that happened, haha). If we leave for vacation in the winter, I just drain the tank.