What is the lowest rideable gear ratio (chainring/cassette) possible without feeling like having to balance/trackstand? I'm doing some virtual simulations right now and I'm just setting up IRL limits. If it differs on road and mountain tires, I'd like ratios of both.
One issue is that it can be harder to balance a bike while spinning like crazy than while pedalling more sedately. Timing a gap in traffic, for example, I can get below 1km/h, but changing right down is a mistake because when I want to get going again I'm trying to push 100rpm after a couple of strokes. This is worse if I take the MTB on road, because of its smaller small chainring - it has very limited use.
Another is that gearing too low produces too much torque, which is worse on steel uphills. With your weight on the saddle or over the back wheel, the front wheel will lift, but with your weight forwards the back will slip. I've had this on trails (MTB) and wet roads (tourer). You have to push quite hard to get up enough speed to get through the weakest part of the stroke (when the pedals are vertical) so simply starting gently isn't an option either. Because of the grip, this heavily depends on the surface, so it's not just about the bike.
One bike I ride has a lowest forward speed at which I can balance of around 4.3 km/h. Any less than that and it gets too tippy. These speeds are best attained going up a significantly steep grade of 10%+ and its a lot of concentration to react in time.
So to answer your question, what's the slowest speed you can ride at while maintaining your balance? Figure that out, perhaps on the flat and on an uphill, then work backward from there.
I feel this is one of those questions where the asker has an idea, and formulates a question. But the answerers can't see why the question helps the idea.
Instead, consider expanding your question to show your ideas and your end goal. More minds may help you get there better and faster.