Massive Thermo Mass Battery Suggestions

Ok here the dillieo… Looking for aditional Ideas and or advice :

I am in the middle (ok 1/3) of my round house build.

I really want a rocket stove mass heater in it but no way of getting it approved!
So
I am going to make the most efficient rocket stove boiler the world has ever seen! well maybe just my county anyways? Ha!
It will be self circulating, tankless/thermo mass, multi fueled & or chip feed & it can be manual fed normally when powers not available!
I have my detailed plans for boiler all figured out! I’ll have time next winter to build it,

I am getting close to putting in the hydronic heated basement floor, Soon! Spring is near!

I want to install a “massive” boiler heated Mass put in the center of my 36’ round floor! My ideas so far;

Dig say a 5’ deep and 10 x 10 ft hole, Would that be big enough? or even bigger maybe? 3 storey, 3k sq ft

Insulate with ?? heavy straw cob or Styrofoam or ? Not sure what could handle that much weight? Any suggestions?

Then maybe the bottom (if needed) and all sides with an additional insulation of 4” perlite mix that I’v read you guys talk about! Would that be good enough for a slow drain off a mass of that size, ideally @ 200 degrees?

My biggest question is what would be the best & cheapest materials to “pack” in the mass around the Pex tubing? I’m thinking crushed concrete (not next to pex), river rocks and & sand maybe? I need typical materials form a gravel pit! My land is mostly sand…

I do have a pond down the hill?

How about a hot tub?

Dale Look wrote:How about a hot tub?

I really don’t want a water tank? I have several reasons., one of my boilers I made had the water heats separate in as much as 100 degrees in 18" when uncirculated. A much larger very short long and wide tank would be needed, Steam would need to be vented outside, water levels monitored & I am going to pressurize my boiler with the goal of using as little antifreeze/water as possible. I will only be bringing mass up to heat once maybe twice a day. Unoxidized stagnant water gets nasty pretty quick. The water would need to be cured & piped every were. I am thinking mass is much better for me?

Make the house with a wood stove right in the center with a chimney, but build it so you can take it out and build a rocket mass heater in the center after the code inspectors and insurance people are long gone.

PS house looks cool.

What gets me about these approvals, or lack of them, is that you can buy crappy leaky inefficient stoves (from China) from one of the box importers, and fit them and that's ok. If you try to fit an efficient,clean burning and relatively safer RMH, you can't because it's not approved.