I've posted several times about lithium ion batteries, but people's beliefs about batteries seem to border on the fanatical with no room for the facts. So it's fine if I don't change your mind, but perhaps someone reading this will consider something other than lead acid in the future.
I would go with lithium ion, but I'm worried about how long it can sit and thermal runaway.
I would suggest you have some misinformation. Lithium ion batteries, including the iron phosphate cathode type (LiFePO4) used in automotive starting applications, have far lower self-discharge rates than lead acid. My Chimera LiFePO4 battery can sit as much as 9 months before needing a charge ... no lead acid I'm aware of can do that.
Additionally, LiFePO4 batteries don't create oxygen when they burn, so they don't burn as hot or "run away" as you describe it: those are characteristics of lithium ion batteries with cobalt cathodes, like the batteries typically found in cell phones and many other consumer devices.
Right now, the arguments against LiFePO4 batteries for automotive starting are (compared to lead acid)
- higher cost
- lower total capacity
- lower maximum current
- charging system voltage differences
- requires protection against overcharging and overdischarging
If you can cope with these disadvantages, the advantage of a LiFePO4 is a typically smaller form factor, low self-discharge rate and dramatically reduced weight.