Unless the bike is designed for a big wide seat with a seat back - i.e. a recumbent bicycle.
I agree, this is important. If you move to the right to let a car pass, they’ll pass with only inches to spare. Occupy the middle of the lane and they’ll change lanes to pass you.
Being in the center of the lane also makes you more visible from car drivers waiting to come out from side streets and parking lots.
Unless you are going up/down some significant hills, you can probably get away with leaving the front (left shifter) gear on “2” (the center gear) and adjusting the right shifter (rear gear) so it feels good / you can keep a decent cadence (peddling speed). Just like a car, it is easier to start in a lower gear (chain on a bigger rear gear) and shift up (to smaller rear gears) as you build up speed.
Brian