Gravity-fed pipe sizing & fixtures

New here. I recently bought an off-grid house, and I'm trying to figure out what to do by way of plumbing. The previous owner had a 300 gal skid tank in the attic, filled with a generator-powered well pump, and gravity feed to the bathroom. The tank has a minimum of 6' of head to the sink, tub, and toilet and 2' to the shower.

This much will stay the same, at least for now, but I will be redoing the interior piping and fixtures. (They froze and split as we were going through closing.) Coupla questions:

--Are there any rules of thumb for pipe sizing in gravity-fed systems, as there are in pressurized ones? Working through the pipe-sizing tables in the 1940 BMS 66 "Plumbing Manual," I came up with 1-1/2" from the tank to the first tee and 3/4" for the branches. Anyone else have experience with a similar system?

--Seems to be relatively easy to find 3/4" full-port ball valves for in-line use. Does anyone know of a source of *faucets* with 3/4" connectors and full-port ball valves, rather than the standard 1/2" globe valves, which are pressure hogs?

--Last, I am looking for a nonelectric hot water heater that will function at very low pressure, if such a thing exists. Would settle for propane, but the Bosch 330 unit seems to require at least 15 psi; freeze-proof solar would be better.

Thanks!

Most just do what is cheapest/easiest with very little attention paid to pressure drop. 3/4 is common because any bigger gets $$$$ fast.

Last time I "created" a faucet, it was with 3/8 full-port ball valves. They do better than 1/2 globes.

If you are OK with plastic, use PEX and no Tees--put a big manifold off the tank and 3/4 individual lines to each fixture with gentle bends everywhere. That is cheap, easy, and minimal loss.

There were a couple threads on here about low pressure on-demand water heaters, but I can't find them now.

Thanks. That's pretty much what I've done already, temporarily, though I used 1/2" tubing instead of 3/4". Cold water in the sink is fine, and the toilet fills slowly but I can live with that. Haven't tried a hot water heater yet, or the shower. I may try a bag shower or something like that.