I think your (Helga's) idea of agrarian and now modern industrial 'civilisations' as essentially 'experimental' is about right. As an aside, some of the apparent pre-requisites appear to have interesting histories. I understand for example, that writing and arithmetic have been invented many times, and pretty quickly at that, to match any rising trading trajectory (following from storage and transport), whereas alphabets and wheels were perhaps rarer inventions and needed to be handed on. Some civilisations missed out!

There are, despite the pattern of rise and fall, some agrarian civilisations that maintained, albeit erratically, high density populations on the same land for very extended periods. Some were maintained over thousands of years to this day, and they shortened fallow 'soil restoration time' , which is as you say a key limiting factor, mostly by re-cycling a high proportion of soil nutrients. These populations also were notably, but not always, associated with 'renewable' deposition from large rivers; the Yangtze, Indus, Nile etc. and became Empires, or part of Empires. (Rivers could have been a long term sources perhaps of phosphate and potassium and carbon?) In those situations the ratio of the size of the 'superstructure' compared with the agrarian base would be severely limited by the rate that the base could renew itself. By superstructure I mean the layers of society supported by the farming base; everything from specialised local craft industry and its trading, through to regional food storage and transport (insurance against inevitable variation in crop yields), and finally political and military elites. The limiting factor for agricultural production, and in particular its 'export' to the superstructures, seems to have been the rate of renewal of soil nitrogen.

A 'new organic' arable farming, however, emerged during 17thC that shortened the fallow period for temperate agriculture. Soil nitrogen levels in parts of England are calculated to have been raised 3 fold by the use of clover's N fixing power, so that after 1750, England's 'carrying capacity' was raised from about 6M to closer to 18M, when by 1850 22% of the population in farming could just about feed the rest. The fast growing cities of course by that stage were outgrowing domestic food production, and the British Isles soon needed to import the majority of primary calories, which is still the case.

Large areas of North American farming, however were not able to achieve sustainable economic sale of food to the cities without mining the centuries of pre-farming soil fertility. According to Geoff Cunfer's expert study (2005) “On the Great Plains”
“They applied manure as it was available, rotated legumes when it was convenient. But they had no strategy for the very long term. By the 1930s, … soil nitrogen was about half what it had been at sod-breaking and crop yields declined steadily. … Soil nitrogen and organic carbon drifted steadily downward, and with them yields and profits. Faced with this dilemma, farmers … [with industry] … appropriated abundant cheap fossil-fuel energy to import enormous amounts of synthetically manufactured nitrogen onto their fields.

The GTI is a very good choice .. I'm curious to hear your impressions some four months later.

I hear dealers will often clear out inventory at the end of the month to meet quotas; it sounds like waiting was the right thing to do for you.

If I catch you in the atlanta area and you don't mind, I'd enjoy a ride in your vehicle