Well, first question is "Is shading an issue where you go?" In my case I live in trees so I ended up going 4p through a combiner box so I can keep each panel running independent from each other, but it was no big deal for me to run 8awg wire down the size to my distribution box. With 8 panels up top you can either go 4s2p or 2s4p quite easily. Being mounted flat on a roof you're always going to take a hit in production because unless you're on the equator at solstice, the sun is shining at an angle on the panels instead of straight on.
You can see my
process and what I did on my camper in my other thread. It might be informative.
OK, so let's do some napkin math with rough numbers:
100w panels are usually in the region of 22Voc, 20Vmp and 5a, so we'll use those numbers for the guesstimates. The rule of thumb for 12v systems is you need 10a of SCC per 100w of panel, so you're going to need somewhere in the 60-80a range. 60 would probably be fine which are pretty available.
8s: 176Voc, 160Vmp, 5a is pretty light load so 16awg would work, 14awg would be better. This lets you re-use the wire coming through the walls without having to re-fish anything. Down sides are that you'll need a really expensive MPPT to take advantage of that high voltage, and if a single panel gets shaded your whole system is nerfed to the lowest panel's production. Works fine in the desert, pretty worthless anywhere else.
4s2p: 88Voc, 80Vmp, 10a so 14awg would be fine, 12awg would be better. You
Might have that size already in the wall so you can re-use that wire. No combiner box needed, just a Y-splitter which makes wiring easy. Pretty much any half decent brand of MPPT controller will work well with those specs. Downside is that if a single panel gets shaded by a branch or anything, half your panels are nerfed and not producing squat.
2s4p: 44Voc, 40Vmp, 20a so you'll need at least 12awg wire, 10awg or 8awg preferred. You'll need a combiner box or a stack of fuses (which introduces multiple points of failure) which adds about $100 or so. Again, any half decent brand of MPPT will play nicely with those voltage and amperages. The plus side is if a single panel gets shaded, 3/4 of your strings are still plugging along just fine. The down side is the requirement for an extra piece of hardware (the combiner box or fuses) and the extra cost of the thicker wire.
8p: 22Voc, 20Vmp, 40a, so 8awg really good silicone wire, or 6awg thicc boy wire. You'll have to fish expensive thick wire through the walls, you'll need an 8 position combiner box ($$$!!) but if a single panel gets shaded you're other 7 are plugging right along. May have problems reaching starting voltage in cloudy weather. Just don't.
3s/4s for 7 panels: you'll have to run another set of wires through the roof and get a 2nd SCC involved because you can't get 7 panels to parallel up. At that point you have 2 arrays, a 3s and a completely separate 4s array with their own wires and their own controllers. That's why everyone is trying to get you into that 8th panel.
OK, those are how the different configurations work out with each other using example math. Does that help make sense?