Dont come off the M58 at Switch Island, come off the M58 at J2, turn right down School Lane, over the railway bridge, left down Deyes Lane and then there’ll be somewhere there to leave your car and jump on the canal near to Maghull Station. It’s not a bad area so it will be fine.
You can pick up the Leeds to Liverpool canal within a few minutes from Maghull Train Station (Maghull Square) and you’re away! At commuting time, Bootle area will be quiet! Certainly in the morning i’d imagine.
Or you could come down the East Lancs Road from the M6, Park around Kirkby / Knowsley and ride down the rest of the East Lancs Road into town. I think there’s some cycle paths / lanes but not sure on the whole route though.
Maghull / Canal is best option i’d say.