Sunshine , thinking ahead here , i looked at your floorplans and i reckon that your best way forward would be to keep the 125 and add the 210/220 to it , my reasons are that i have seen your vids and a lot of your work seems slopey and rural , this could be a prob with a 350 having the large footprint and only one option to site the tank outlet , i run a 350 and a 175 , i find the 350 a proper pain at times due to my parking options , and im pretty much city based .
What i do is run the 350 for as long as i can then run the 175 with the smaller footprint at the end of the day , in doing this i can get the most from both tanks , it also pays to put a tap on each going to a T splitter with one feed to the pump , that way you can decide at any given time to pull water from either tank , and you will still be able to circ the water in your 125
Rich