For your next seatpost upgrade attempt, you might try inserting a second steel tube inside the new post for reinforcement. A steel tube of 150 cm or so in length would likely work, since the unreinforced section of the seatpost would be high enough above the frame to lessen the likelihood of bending. If you're able to find a correctly tight-fitting tube, I suggest epoxying it in place inside the seatpost.
By the way, the folding bike you posted above looks as if it has a seatpost that's larger in diameter than 25.4 mm. Other things equal, and oversimplifying a bit, the strength of a tube increases as the cube of the diameter, so even a small increase in diameter makes a big difference in strength.
[Edit] Forgot to mention: if inserting the 25.4 seatpost resulted in scratches in the finish of the seatpost, that means that the top of the frame tube into which the seatpost inserts has been damaged. Not uncommon. If the frame is steel, you can probably safely pry the binding ears apart to where they were originally if you're careful enough.
If the frame is aluminum, might be safer to file the frame opening until it no longer abrades the seatpost.
If you're not sure whether the frame is steel or aluminum, see whether a magnet, such as a refrigerator magnet, sticks. If it does stick to the frame, it's steel. If not, it's aluminum.
Last edited by Trakhak; 06-02-22 at 03:59 PM.