Power supply transformer question, double primary
simulate this circuit &ndash; Schematic created using CircuitLab
Instead looking for the transformer you can use components already pre-made, like isolated RS485 transceiver module, MCU module, lastly your custom metering module with isolated DC/DC converter and signal isolator IC, like ADUMxxxx
ISO RS485 examples:



The black bricks on photos are isolated DC/DC converters, while the ICs are digital signals isolators TX,RX,RTS.
The MCU can be any kind of it, for example Arduino, Raspberry PI, ...etc. The SMPS you can buy in any electronic store, like Meanwell, Recom, TDK Lambda, Traco, ...
Meanwell EPS15-05:

Then you have to choose a small DC/DC converter 5V/3.3V let's say 2W, example Murata MEJ2S0503SC (instead of custom AC transformer):

Then you need a digital isolator like ADUMxxx or Sixxx for SPI or UART commuinication between your custom board and MCU.
Similar metering application (3 phase) SmarPi.

It has an input SMPS, which supplies RPi or Arduino. Through a digital signal isolator and DC/DC converter (all in one) it supplies and communicates with metering IC.