You are right to be concerned. The issue is not the average draw on the hydraulic pump, but the max. Think about the boat trying to broach down a wave at full speed. How much weight does it take on the helm
to turn her? The pump will try to do that. I use a Simrad
RPU-160. it CAN draw approx 42amps at stall. Of course it almost never does! But it can...
So, the required pump must be matched to the ram, otherwise the steering will be too slow - it's got to work quickly enough to steer properly. This can be calculated (volume of the pump (at working press) v volume of fluid the ram needs to put the helm
hard over, v required time to hard over). Once you know that, and the stall current
of the pump, you can see if the AP computer can supply it. In my experience it's best to go for the biggest pump that the AP computer can supply.
If the AP computer is too small, yes, you can absolutely use external switching. Don't use relays though, use FETs - MOSFETs to be more specific - they are like a solid state relay, and there are no contacts to wear out. Most relays are only good for a million or so switch cycles, which is not a lot in this application.
I did this with an older Simrad
AP which suffered from the lack of AP computer power problem. Worked fine, but eventually the AP was replaced by a better one.
There is a commercially produced controller (used for robots) that can be bought online that will fill this function nicely. Let me know if you want details and I'll look it up - I've forgotten it's name, but I should be able to find it with a bit of google's help!