Hello thanks for your encouragements and suggestions, sorry I had no update this week I had no progress to share,
work and also the "normal" tadpole trike broke on me that was a bit dangerous - there was an unique M8 screw to serve as an axle for chain pulley and support the seat, so when it broke the seat sank and wobbled, and the pulley and guide tubes blocked everything.
I had fun extracting the broken screw from inside the frame and cleaning up the threads ...
So back to the pyhton trike, I welded on the posts for the rear V brakes, and painted the frame blue
I thought the color was an essential element to improve steering stability ... at least it is sort of reassuring to finish something.
I also fit an ugly but removable carrying box, the rear frame was sized to accept a standard euro box 60x40x42cm for loading.
weight in the back did not change much the steering but it was kind of expected, the CoG is hardly moving by putting 10kg in the back.
On a positive note I noticed I am getting used to pedalling without causing too much pedal steer, by pushing slightly towards the outside.
As you said also getting used to steer with the legs, they are way stronger than the arms and determine steering, arm restance is futile.
Uphill here in the mountains is fine the trike is like a tractor, with the MTB gear ratio on a 20 inch wheel it climbs well but slowly
Uphill is also very stable in direction, I think because the weight is behind the front whell traction, so whatever trail / angle it works.
Downhill (very gentle like car park 2% slope) I tried "free caster front wheel" as you suggested, not reassuring
The way I found to stabilize steering downhill is stop pedalling and hold one leg straight against the frame, acting like a damper and steering.
I have to try a steering damper next as suggested earlier.
Thanks for the wheel size idea I didn't get it at the beginning that makes sense I can effectively change pivot angle and by putting in small whells, I ll try that I have made a quick drawing and your suggestion would allow to change angle from 62° to 58° with 16 inch wheels, nicely it does not entail modifying the frame.
The steering comes apart to take out the front, bearing cups need to be tighter than on normal bike because there are axial and radial forces here, those bearing cups may wear out quickly but wanted to reuse bike components / critical welds of a MTB frame, and the head tube is not easy to weld.
In retrospect I think your "rod end bearing" solution is more efficient & modular , easier to take apart and modify if need be.
I also supect that the slight friction of those bearings ( they have a bushing and are not roller bearings if I am correct ) may help with steering "nervousness"
Finally one thing that is sturdy and practical is the seat mount, made with dropouts :
I think I will try to lower the seat by another 5cm or so.
Thanks again for interest and suggestions, the kids bike rear wheels are worth trying out and I am way beyound ridiculous at this stage anyway