Trim tab - yeah, that's a fair summary. It will however have negligible effect at the speeds you're talking about. As said earlier in the thead, best thing for a long slow cruise is to use small wheel movements and soon you'll find out just how slowly the boat responds. Overcorrection just exaggerateds the problem. I did a cruise with an aux a while ago to basically give it some excersise & blast the spiders out the exhaust
. Reaction times using the main engine as a rudder .... 10+ seconds! (doesn't sound a lot untill you are steering a boat!) Even without the "vectored thrust" of the main engine prop, small wheel movements were all that was needed.
I'll need to hand you over to someone with PTT for the trim question. I'm still on manuial trim, so it gets set for the sea conditions before I launch - I just have to put up with where I set it for the fast stuff.