I really like your new avatar. My minds been blown for years now. We have company now so it won’t be until about Friday that I’ll be able to resume working on the frame. My son in law named the bike. He took one look at it and said “it’s a Death Trap”. I’m relaxing now, resting on my frame building laurels.
These inexpensive bicycles have steel that welds real easy. Maybe they are made out of mild steel? Your lateral brace needs to be down, closer on the head tube to the down tube. They were not high up like a mixtie. Of course it looks like you are thinking in 3D and using the cables to hold it in place. Flatten the tube in a vice and grind off the back so the front non ground part is on the outside of the drops. the back fits tightly in the drop space, Grind it so the part that spread with squeezing to flatten is gone, no expanding out but a nice taper and parallel line.