If I commit to tanks, which I think is probably my only option right now, knowing that I have a 12 turn GA starting right now (my first), am I building wealth/research to squeeze every last drop of tech out for as long as possible until I can actually build tanks?
Yes, except for building production infrastructure (forges, factories, plants, good levees, and some health buildings to not starve) and if your safety is threatened. Try to get two GP for a second GA.
Basically, what you want is reach Assembly line ASAP, industrialize, go for tanks and build tanks. Flight would be nice, but is entirely optional, and can safely come later. By later I mean after you start the war, to airlift some reinforcements.
You do not need any other infrastructure, except maybe the odd library in good commerce cities you conquered.
If Zara does not attack you, build wealth (or science if you break even at 0% gold), and only the above infrastructure. Workshop/Watermill anything except towns and essential farms now. Workshop towns, except maybe your capital, after reaching Industrialization. With SP any city that has a few workable (preferably flat) land tiles and a minimum of food has the potential to be a production city. As a rule of thumb
in this situation, I would say that 5 workshops make a decent production spot, and such a city is worth it to be grabbed now.
Regarding your Navy, relative strength will only matter when at war.
islands are already half settled