That motor should be operating between 5300 and 6300 RPM at WOT. Most recommend leaning towards the higher end of the range, particularly if you are at the lower elevation range of where you'll be fishing (you'll lose power/RPM as you go higher).
You should pick up about 200 RPM for every inch of prop pitch you reduce. I would drop the prop pitch 4" to 5" to hopefully get to around 6K RPM. This should also help with the prop torque or "lean" that you are experiencing. A clockwise rotating prop lifts the starboard side, lowering the port. I have a 300 HP outboard on a boat with a fairly deep vee. I get some prop torque or "lean" to the left at certain RPMs and trim positions, but it is not so bad that I feel the need for trim tabs. Just a function of single-outboard motors on boats with less horizontal stability (due to the deeper vee).
As for sharing "house" and "starting" batteries, your gut feeling that it won't work long term is probably correct. The new fish finders these days pull a lot of power, especially if you have FFS. Add to that radio, nav lights, electric downriggers, and any other draw, and the battery may not last very long. I have my starting battery on an isolator from my house battery, which means once the starting battery has 13.6v, it sends any additional power (via the alternator) to the house battery. This is a one-way relationship. This way, even if you run your house battery dead, you always have a fully charged starting battery.
You should pick up about 200 RPM for every inch of prop pitch you reduce. I would drop the prop pitch 4" to 5" to hopefully get to around 6K RPM. This should also help with the prop torque or "lean" that you are experiencing. A clockwise rotating prop lifts the starboard side, lowering the port. I have a 300 HP outboard on a boat with a fairly deep vee. I get some prop torque or "lean" to the left at certain RPMs and trim positions, but it is not so bad that I feel the need for trim tabs. Just a function of single-outboard motors on boats with less horizontal stability (due to the deeper vee).
As for sharing "house" and "starting" batteries, your gut feeling that it won't work long term is probably correct. The new fish finders these days pull a lot of power, especially if you have FFS. Add to that radio, nav lights, electric downriggers, and any other draw, and the battery may not last very long. I have my starting battery on an isolator from my house battery, which means once the starting battery has 13.6v, it sends any additional power (via the alternator) to the house battery. This is a one-way relationship. This way, even if you run your house battery dead, you always have a fully charged starting battery.