Boat trim - am I missing something?

Well, your holding tank is near the bow.......so there's that weight up there.

If the autopilot is steering too much, then it is because you need to adjust the sensitivity. You control it's responsiveness based on how you set it.

Which autopilot do you have? I've been able to set the sensitivity on all of them. The reason you're boat benefits little from trim tabs for speed is because that boat is too bow heavy. Like the other poster said, he runs an extra 100 gallons in the stern tank to get the COG aft. Having too much weight forward is also the main why they turn back and forth and ride like they do in a following sea. They got the 50' Post right after they changed the tankage in the early years of the model and it doesn't do that.

By "close sets" of waves, do you mean a few waves completely different from those depending on the sea state, (as typically happens with navigation waves from other boats), which you must cross briefly before returning to the "normal" sea state?
If so, you will never find any a/p capable to anticipate them.
Of course you can do a better job in that situation, because you can see those waves approaching, while the a/p can only react as soon as the boat is affected by them.

But other than that, if the a/p makes the boat behave as if she were under PIO conditions, I must reiterate my post #17.
In fact, I fully agree with CJ: I also never came across any a/p where it was not possible to tune the gain manually - though I did come across several where doing that is somethat tricky.
Did you RTFM?

We were looking at Post years back, they finally moved the fuel tanks aft In the mid 90's which help the ride a lot. Any Post prior to that had a tracking problem. Talked to a few Post owners that said they are squirrely in following seas.

For my 46 with the older style hull and lower Hp, it's not a following sea as much as a quartering sea.

In either case , I raise trim tabs to bring my bow up and if I can speed up just enough to get ahead of the wave sets, the ride is much better.

A stern quartering sea is still more work than a straight following sea though and I too have changed course for a more comfortable ride.

I do not have a tender on the bow, which I think really hurts their performance.

I can see a speed increase by bringing my bow down going into head seas of between 0.5 and maybe 1.5 knots depending on conditions.