Yeah I wouldn't recommend mixing cells on series, well you could, but the pack will only perform as good as the worst Ah cell in the string. On parallel, I'm not really worried if 2 packs would perform a bit different, it will just come out in the Ah capacity as different (like having one lung that's smaller than the other, you would really notice it in your breathing).
Will actually recommends capacity testing each cell individually before building a series pack, so you have a predictable benchmark for your pack, then if you have some extra cells laying around for example, you could mix and match cells to find the most like-performing cells to go in your pack. Generally, battery cell vendors (good ones anyways), are supposed to be matching cells that are pretty close to identical in orders when you get say, a 4x or 8x or 16x pack)...
Or if you don't just happen to carry a 'handful' of spare cells on your shelf (like I don't), then at least you'll have an idea of what you can expect out of the pack, because you know the cell with the lowest capacity, and if you find 1 or 2 cells that are really far under spec, you can replace those cells or get the vendor to replace them, if they aren't performing to their advertised spec.
I would rather just decide on a voltage you want to run, stick with it, like if it's 48v then just buy 16x cells and build a full pack. Then later build another and parallel it, etc...
On a side note, for my most recent 12v RV battery build I went with 8 cells from these guys:
https://www.aliexpress.com/item/4000948488435.html
Back when I bought them in October last year they were running around $1050, seems the prices are a bit higher these days. Back then I saw some vendors selling 8x 280Ah cells for as low as $800-ish, I bought these ones for $1050 because the seller had good rating/reviews and they used better quality copper (flexible) bus bars (mine look just like the ones in the pictures there).
