What gauge should I use
I think maybe I can help out on this as a lay person...
Sounds like you might have some confusion over wire gage lingo, I kinda went through this confusion myself.
Wire gages go down in size as the wires get larger. Most often DIY projects will deal with 15 amp and 20 amp circuits that typically use 14 gage and 12 gage copper wire respectively. You might get into 10's 8's and 6's for 240 volt appliances.
Those of us crazy enough to attempt panel feeder type work will be the ones that run into wire gages heavier than 2 gage.
So wire sizes logically go down to 0. When the wires get bigger, nomenclature used to be that you'd get more zeros. You'd go to #0 gage then #00 gage then #000 gage then #0000 gage. Now the lingo is to call a zero a "naught" and refer to the number of "naughts". #0 is called "one naught" and written as 1/0. #00 is called "two naught" and written as 2/0. And so on up to 4/0.
4/0 aluminum is the size needed for 200 amp service. Apparently larger sizes are referred to in mm diameter, but anyone not an electrician is unlikely to be daling with that. For my jurisdiction, I'm not allowed to do a service larger than 200 amps as a homeowner. And larger cable than 4/0 is not going to be found at a big box store.
Please do NOT consider any "before" picture of my house as any kind of endorsement of any particular construction method. In fact, you should probably assume that if I post a "before" picture, I am posting it because I am soliciting advice on a proper replacement for one of MANY things done wrong by a previous owner.