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Nice project, but you can connect all these sensors (and more) directly to esp8266, atmega unnecessary there.
You do not need to calibrate if you will use very popular and cheap acs712 module, you can choose from 5 to 30A model, with this sensor you don't need rectifiers and pullup resistors.
Simple design, but if you try lm2576 - you will never use 7805 for outputs more than 0.5A, why !? because of high inefficiency and result is high heat, if input is 12V and 7805 regulator output 5V with load of 1A, then the to220 case temperature without heatsink is (Vin-Vout) * Amps * CW + Cambient = (12-5) * 1 * 69 + 25 = 509 celsius ! you quickly blow out regulator, to handle that power you have to use min ~18C/W heatsink, and is still will be wery hot, about 150C in room temperature ! 7805 datasheet says "max disipation power without heatsink is 1.9W" its only about 0.27A load ! 1.9w / (12v-5v) = ~0.27, check for lm2576, i always use this chip for higher loads (up to 3A)
Great improvement, but there i see some problem, i have CNC plasma cutter to and smoke extract fan, then plasma cuts steel, the melted steel go out and fallig down, steel become cold and in you case when you fill out water, then the small steel pieces will go to water tank and stays there, after some time mall steel pieces fill the water tank and clog the hoses, other disadvantage - is lot off rust in the water, but the health is at first place ;)