Looks great! I'm going back to watch your videos now.
I'm curious. Did you consider having the heat storage in your basement?
I've dreamt of a system with hot water storage in the basement hooked to antique cast iron radiators in the living space gravity fed.
There is no doubt there are advantages to having the system all in the building you are heating. 1000 gallons of 180F water in the basement would be very nice indeed.
In my case there was no way to get 2 x 500 gallon propane tanks in my basement with out excavating and knocking out part of a foundation wall. Also the fact that one of the reasons for installing
a boiler system is to heat multiple buildings which was my goal as well.
As far as gravity feed, I've heard those systems can be effective but definitely not practical in my case with the design of my house (four levels split, first two on concrete slab). Ideally I'd have radiant floor though out, but unless the house is designed for it from the start it's rather hard to retrofit - especially with fully finished ceilings on the first two floors (that would have to be ripped down to install radiant tubing for the floors above, then there is the existing concrete floors for the basement and next level up that don't have pex tubing)