I'm in about the same situation, having a pre-ST autohelm 3000 that is under
powered and doesn't take NMEA input. Otherwise it works fine in calm water
with the course set manually..

I have already found a solution to the increase in power, however I'm still
searching for an upgrade to the control head.

Be advised that none of the pre ST autohelm/Raymarine pilot will except
NMEA input.. Even the Z80 interface seems to dislike the current 0183
formats.

Then we get into the ST models. Even some of these early ST models still
required an interface to convert the NMEA to SeaTalk before it goes to the
AP..

I investigated a ST5000 on ebay that had a location for the NMEA connection
and the markings were molded into the case, but the connector pins holes
were empty.. Must have been a very early release..

Then there is the problem of your drive motor clutch.. I believe that the
ST5000 is the first model to have a clutch control in the control head..
Here again, I would approach this with caution since there are going to be
control heads sold for tiller pilots that don't require a clutch release.

I would recommend staying away from the current generation since the control
head don't necassarily have the course computer in them, just some secondary
circuits, display and buttons. The course computer is incorporated in some
other nav station equipment..

I think your approach is correct as you have evaluated each of the system
components and determined what to keep and what to abandon.. Your boat size,
like mine make it nearly impossible to purchase an off the shelf AP system..
My boat being 13.5 ton and tiller steered.. Even the largest tiller pilot
system comes up short.

Good luck.
--
My opinion and experience. FWIW

Steve
s/v Good Intentions