I noticed that when I looked ad the design that there are a few points that need strengthen.
I make it 3d for the images but it is more for a reference for how it looks and how much material I need.
The low one will get a seat suport what creates triangles what makes the frame stronger.
I will also ad plates and struts. On bycicles the often make the place where it is welded thicker to make it stronger. I will do the same. The frame will be a bit heavier but also a lot stronger.
The plates to mount the wheel in the back and all the disk brakes will also be thicker.
Before I start building, I will calculate for what I need and where the weakness is in the frame.
It is over 18 years ago that i did some welding. But before I start, I will try it first on old parts that I don't use.