Not a practitioner of the SFG method, just a gardener.
General observations: Location, location, location: taller crops need to get planted where they will not shade out the other plants - unless of course you can actually USE the shade, for say, greens during the heat of the season.
As already mentioned, and you noticed, access to the plant is important. I have 3 tomato plants (OP) planted right smack-dab next to each other - maybe 16inches?, but I can get to all sides of all 3. Peppers are on the south side where, today, they are protected from a ridiculous high wind by the wrapped tomoato cages. I have four peppers, about 12" apart. If they seem to be too close later I may pull every other one or try to TP.
Greens can get tucked in anywhere, so if you are pressed for space, it's a bit silly to reserve a row just for lettuce. Put between longer crops like cabbage or broccoli. Wide-row or SFG makes sense, it's how Nature does gardening in the big 'ol world.
I like the idea breaking up the space a bit. 4ft is about the widest you can reach across, so at a minimum, one additional walkway would be good. It doesn't have to be any wider than a footpath - just enough to stand in/walk on. Two paths would be ok - 3ft beds. The space you are giving up to footpaths is not "lost" because it's an investment in the health and care of your remaining crops.
Some varieties may not like being crowded more so than others, so maybe your spacing was OK for other varieties - but it sounds like maybe you need to increase by 25-35% esp. peppers. Last year I had planted mine (baby bell, banana, jalepeno) about 16 inches apart, and the plants themselves weren't giants but I got a decent crop of peppers!
Do some more research into wide rows and SFG, and good luck to you!