Yeah we can get off track easily.
Sounds like the big question is how much better will the solar be with shorter wires? Longer 120 is not a factor under 50'.
Plan exclusively for the future 400W system. Then do your voltage drop calculations for both ways. https://www.southwire.com/calculator-vdrop At 10' 6AWG is probably good, but 4 will be half the loss. What can't be calculated is the value of your cash and sweat.
Moving LI batteries inside is a no brainer for me, if you plan on any cool temps. I've been snowed on in mid summer! There are heaters for batteries but that takes time with the truck noise, fuel and fuss.
