board to board soldering without castellated vias, how?
You're doing it backwards if you're trying to SMD-mount these modules:
You get Wireless and high-speed boards with castellated edges, because it's easier and cheaper to make one small board with many layers, on-board antennas, impedance matching and so forth, instead of directly putting your components on your main board.
These cheap voltage converters do not require any of that. Instead of buying a board with standard components soldered to it and soldering that to your motherboard, simply directly put the voltage controller IC and the other components on your board. The datasheets and design assistants of the voltage regulator IC manufacturers are good, and it's pretty easy to do so.
So, long story short, the thing you want doesn't exist because it bears no advantage.