We assume that CO₂ never previously reached these levels in previous cycles. I don't think ice core data would necessarily capture higher levels once temperatures reach a certain level

Just a thought: Where did all that CO₂ come from and where did it go to? The amount of CO₂ that can be released on earth is very limited. New CO₂ was not produced very often(vulcanism may play a role here) in the history of earth. Most of the carbon on earth was bound in carbonates and in fossil fuels. Also the newly produced CO₂ has no way to fastly disappear somewhere. The process of creation of new sediments or new oil takes ages.

Currently we are burning a lot of coal and oil, so we can be pretty sure that most of the CO₂ in our atmosphere was in fact created by humans. And as I said CO₂ needs a long time to be reabsorbed and formed to new oil/coal. Therefor we shifted the whole CO₂ cycle(wether it was above 400ppm previously or not) by another 100 ppm(which would lead to >500 ppm peak following your theory and 300 ppm low leading to an increase in 100000year average temperature of 1°C). This shift might reduce the strength of a possible(yet unproven) possibility of another little ice age.
But the problem is that the >1°C shift upwards won't disappear after the ice age, but will stay for millions of years making earth hotter than the current ecosystem can withstand and did withstand for the last millions of years.

Once the temperatures rise to a certain level, we will get increased water vapour which as you pointed out will reflect sunlight.

The problem is water vapor will only reflect sunlight when it recondenses in the atmosphere. This process requires lower temperatures, so I wouldn't be to sure about water vapor regulating everything.

When temperatures are hot crops will grow (with sufficient water).

Yes they do, but where do you get all that fresh water? Fresh water is to precious to be just evaporated in an age of heat.
Also many freshwater sources still contain some ions(including salt) that will slowly destroy your soil when all water is evaporated.

The reason we see so much about carbon and climate change is that there are people making money just talking about it and I don't think it would make a positive difference anyway.

Yes. Sadly most people work for money rather than for their children's future.

I never heard about Algae farms, but they seem to be an interesting solution.