Marek and Stefan, I do understand that right now you may be controlled more by what you have, and to an extent what is available on line in these weird times.
Please accept that I may be offering thoughts of what would be nice. You mention 20 by 2 mm, yet your drawing showed 3 mm, 2 is a whole lot better I believe. I do keep an eye on what has been put out at the side of the road for the [people] to get, sometimes there is stuff I can use! BBQs are being thrown out round here, lots of sheet metal.
Are you able to access any of the racing car design books? Costin and Phipps "Racing and Sports Car chassis design" while now over 50 years old is still a base point for steel chassis design. There is a lot in there about bracket making, diaphragm frames and such.
Likewise I know that you now have time and want to make progress, if you can, just do a bit more research of lighter ways to do things, more complex but you will make a car you will be proud to show off as you will anyway.
As an example of where I'm coming from, have a look at the Austin chassis you have. It's lasted nearly 100 years. Then have a look at the approx 1950s steering wedge and think if, with the knowledge you now have, and equipment, could you do something less blacksmith. Again, no criticism of the bracket, it's what was done and actually promoted in books of the Austin special builders in the 1950s, can it now be done differently?