Are there ways to minimize the area of my septic leach field?
I know you said "However, I insist on a simple system relying on gravity and no pumps", but I am going to ignore this and suggest you consider a mini-sewage treatment works.  

We used to own a house with a Klargester biodisc, and it ran without problems for the 10 years we owned the house.  The outflow was clean enough to discharge into a ditch.  One of the advantages of the Klargester is that there is no leech field to get blocked, and if the pump does need to be worked on, it is reasonably accessible.

We did have a problem with the electrical supply (which I fixed by replacing the wire with an armoured cable).  Before I did that, we had to switch off the Klargester when the ground was very wet (otherwise the RCD/GFGI tripped); this means I know that leaving the Klargester without power for a couple of days every so often is not a problem.