Right, this is my initial plan. I wasn't clear when I said "near the alternator" I meant electrically, not just physically. (But to be clear it's the + terminal of the starter relay not the starter.)
And to be a little more clear, boats often times use a starter relay in addition to the starter mounted solenoid. And when I reference the starter, I include the starter motor and solenoid together since this is a GM style v's an old ford style with a fender mounted solenoid, in which case the starter motor actually has a cable connected to it.

The starter relay is on the starter solenoid side between the key switch and solenoid. That circuit was never intended to carry any other additional loads such as ballast pump. There may be a helm breaker in the engine bay that is fed directly off the starter's main B+ post. in theory, you could terminate to the starter feed side of that breaker, but not the helm side. But, you would have to consider the cable gauge. In many cases, its as little as 10ga, but can be as large as 4ga. So, this would bring us back to going directly to the starter main B+ that is connected directly to the battery B+ with possibly a 2/0 cable.

Because the alternator isn't generating this much current at idle or based on connecting to the battery posts? I was thinking about this and I don't know any specs for the alternator. Any idea what it's generating at idle? From experience, if you run 3-4 pumps while at idle, does the system voltage drop to 12V such that you know you're getting everything the alternator has and the battery is supplementing the rest?
Regardless of where in the circuit the load is terminated, the battery is the primary source and the alternator is there to keep up with loads and replenish lost battery voltage. In a practical sense, the higher the potential load, such as 4 pumps pulling 15-20 amps each, the closer you terminate to the battery, the better the entire circuit will be.

The regulators output is based on load, but is also dependent on engine rpm. Even with a stereo pulling 120A, you wouldnt see a 60A alt peak, its just doesnt happen. if your battery drops below 12V with some pumps running for a few minutes, you need to test that battery.

I dont want you to think im downing your use of relays. Its a sound practice and your system in theory, is sound, other then I would suggest terminating directly to the battery. But given thats its going to take 8 relays and somewhere to mount them all, I think its overly complicated, considering a switch with a labeled actuator is readily available.