Since you are set on upgrading don’t half ass it. To get the best performance out of a 24v system you’ll want at least 6ga wire. Most boats now come with 8ga or 10ga. The bigger the wire you use the happier your motor will be. 8ga wire carrying 24v can heat up more than you’d like especially if you work the thing hard. You’ll also experience enough voltage drop to potentially hurt the motor. These Terrovas and Ulterras need to be fed.
You’ll also want to wire in a circuit breaker. That’s just an insurance policy for an expensive piece of equipment.
I’d also suggest some sort of stabilizing mount for when it is stowed. The head, which contains the GPS stuff, is sensitive. By locking it down it’s another bit of insurance for the intricate circuitry in them.
Perhaps the biggest benefit of a stabilizer is that it will guard against accidental deployment. I personally know guys who have been clipping across a lake with just a little bit of chop and their trolling motor comes unpinned and deployed. Their motors flipped into the water and were destroyed at 40-50mph.
Obviously it’s time to get new batteries. Don’t run a mismatched set. Find a pair that are identical and have the same manufacture date. Get the biggest batteries you can fit into your boat.