Everybody seems to have a different form factor for their universal power supply 
I made it easy for myself and bought a bunch of LM2596 boards from China.
But whenever I design a PCB I tend to put the power supply on it, so no messing around with extra pcb’s. My default is probably going to be MP1584. PCB’s with this chip starting from about USD0.60 from china…
About your design:
I do not agree with your decision for 2mm connectors.
I use power adapter boards such as these mainly for experimenting on breadboards, veroboard etc and anything which does not have a 2.54mm spacing is a nuisance.
If you are afraid of sticking components in the wrong socket then mark it on the pcb with a sharpie or similar or make labels in a spreadsheet. ( line spacing of 2.54mm, Adjust text size so it fits, print it, add transparant adhesive tape before cutting it out with a scissors). Glue them to your vero board, or make holes with a needle and then stick a connector right through the label.
Apart from my critique, I like your design. PCB layout looks good, connector with all signals on one side of the pcb, Extra hole for stability / support on the other side. And the connector can be placed on either side. I would place the connector on the other side though by default. After initial testing/debugging access to the components on the pcb is not so important, and if you’re being mooned you have lots of space for silk screen text etc. I would probably also put the I2C address tabs on the bottom side (with addressing comment).
I would also put the silk screen text for Input / output voltages / current next to the respective pads of the connector.
Last remark: Adding thermal reliefs to the connector pads will make hand soldering a bit easier / faster / more reliable.