I'd like to venture that its now a lowest cadence that makes or breaks safety limits,
its the lowest speed where you can maintain balance and steerage way.

I have a hefty recumbent, and my lowest speed up a significant grade is 4.7 km/h. At this speed I'm focussing almost exclusively on bike control, which is staying upright and moving forward in a straight line, and not stopping.

If I turn off the line too much, the bike is much more likely to fall sideways or require a large correction steering input.

Based on https://www.sheldonbrown.com/gear-calc.html your bike will be doing
5.7 km/h at 40 RPM in 34:30 on 28mm 700c tyres.
8.6 km/h at 60 RPM

I'd like to venture that its not the lowest cadence that makes or breaks safety limits,
its the lowest speed where you can maintain balance and steerage way.

I have a hefty recumbent, and my lowest speed up a significant grade is 4.7 km/h. At this speed I'm focusing almost exclusively on bike control, which is staying upright and moving forward in a straight line, and not stopping.

If I turn off the line too much, the bike is much more likely to fall sideways or require a large correction steering input.

Based on https://www.sheldonbrown.com/gear-calc.html your bike will be doing:
5.7 km/h at 40 RPM in a gear of 34:30 on 28mm 700c tyres.
8.6 km/h at 60 RPM, the same.