Rocket Mass Stove/Traditional wood burning fireplace combo?

Hi there. I’m looking for input/ideas/precautions on a possible Rocket mass stove/ masonry heater/ fireplace project.

My house is outside of city limits and I currently use propane to (pretty inefficiently) heat my house in the winter. The house is 2 stories and I have 2 wood burning fireplaces, 1 upstairs and 1 on the ground level. It’s slab on grade and gets cold in the winter. Upstairs can be a comfortable 70° while the 1st floor on ground level can be 50°-60°.

I’ve been considering a rocket mass heater with bench/bells that vents/exits out the current chimney flue on the 1st floor/ground level. Potentially, a bench (bells) could span 24’+ linear feet. The 8′ directly in front of the current fireplace, 16′ along the walls to the right, and a few feet to the left. The general design in this video is the closest to what I was thinking, just way longer: https://youtu.be/0YZCQ9SuQVA?si=5SVUg0VNKXx01DNw

I’ve read that people regret not building enough bench. 24’+ seems like it might be too long but that’s something I’d like educated opinions on.

I don’t think I’d like a barrel for two reasons. 1, safety from accidental touching/burns (I’m having my fiance and her teenage and preteen daughters move in soon which is one of the reasons Id like the lower level to be very comfortable for the kids). 2, Esthetically I think I’d rather go without and just use brick/stone/concrete benches to radiate heat.

My questions and uncertainties:

-Would 24′ of bench be overkill or too long to achieve an upward draft?

-Has anyone ever had a rocket mass heater with bell system alongside a traditional fireplace? Just wondering if you could keep a traditional fireplace tied in for the occasional traditional fire. Not that it’s necessary or even desirable. My fiance likes traditional fireplaces (and we’d still have the traditional one upstairs), just wondering if it’s possible really.

• Why don’t I see any modern or more contemporary looking setups? I’m surprised I don’t see more brick benches with mortar and stone or tile that is a little more modern/ creative/esthetically pleasing. I don’t mind the cob look personally but my fiance is pretty well out. The white washed builds look nice but that’s about as contemporary as I’ve seen.

• How far away from a drywalled exterior wall should a bench/bell be to avoid losing heat to the outside and also risking any heat damage to the walls?

Positives I think are in my favor:

-Slab on grade so I could build it all directly onto the already exposed concrete slab which I’m guessing would help to radiate heat. -I already have a chimney that I can use/tie into. – I’m in the country so I’m probably a little more free to be experimental as long as it’s all sealed up for carbon monoxide.

Any pointers, direction, recommendations on research materials, etc. would be appreciated.