Welcome Fodf ... Can't be of too much help with the newer motors, but I'd stay with no more than a 25 if you're sticking with the single motor and wish to troll.

I ran a 16' Sea-Nymph for years with a 25 and it plenty of go even with a load of crab pots and a couple of passengers but at idle (I know, not the best thing in the world for it), I could troll just fine in the Willamette and other places like the mouth of the Lewis (Vernita for you??).