Ok, maybe this post will help others.
My trim gauge was bouncing up and down at low speeds. Some basic testing suggested the sender was bad.
2014 Sunesta 264 with VP V8-380-C-D. Non-EVC. All online manuals for my DPS-B 2.14 drive called for sender part number 21484383.
I ordered that part number, put my boat on a service rack at the marina, plugged it into the boat's wiring harness to check if it would engage the gauge needle, and it did. You split the harness connector by sliding a red clip sideways and pushing down on a tab at the back of the clip (no photo, sorry). You can temporarily install the new sender by slipping the electrical prongs into the harness socket (careful not to let them touch each other during testing).
After verifying operation, I then rotated the plastic prong on the sender body to the point where the needle on my gauge was full down, and made sure my drive was trimmed full down. That way I would be very close to final adjustment after install.
The following can be done with the drive installed.
You need to first remove the wiring connector off the original harness going to the sender, 31 in the photo below. First, take a photo the connector, clearly showing the colors of the wires going into it. You'll need to duplicate this orientation exactly when reinstalling.
Then with needle nose pliers pull out the red insert inside the connector where the electrical prongs protrude, then with a very slim flat head screwdriver gently pry up on the plastic tabs that are holding in the electrical prongs. Pop off the clip at the base of the connector and pull the prongs out of the connector by tugging on the wires. You'll swap all the connector parts over to the new sender harness.
There is a metal clip, 30 in the photo above, that secures the rubber plug on the engine side of the harness where it travels through the transom. Use long needle nose pliers coming from the side, like in this photo below, to remove and install this wire clip. Stuff some rags in the crevices below in case you drop the clip. You'll be doing that install blind, too, and it's the hardest part of the job (but not terrible).
Once the clip is removed, you can pull the harness out via the drive side of the transom. Unbolting the sender from starboard side of the gimbal ring and cutting the zip tie holding the harness a few inches away will allow you to wiggle the assembly past the back of the gimbal ring and to pull the wiring straight backwards. You may need to pry a little on the rubber plug, where the wiring disappears at transom in this photo, with a long screw flat head to break its seal:
Wrap some tape around the electrical prongs, like photo below, slather the rubber plug on the harness with waterproof grease (like Mercruiser 2-4-C), pull back on the water hose in the photo above and fish the harness into the hole in the transom shield to get it to appear in the engine bay.
Install is basically reverse of removal.
Adjusting the sensor on the ring will probably be necessary to fully dial in the gauge to match the drive position. Rotate it clockwise if you need the needle to go lower (toward down indicator) on gauge, counterclockwise to go higher.
Now, here's the kicker:
While every source online said my sender was 21484383, it wasn't.
After everything was installed, I trimmed the drive fully up and down and the gauge would only give about 2/3 range. Rotate the sender so that the needle was fully down with the drive down, and the needle would only move 2/3 up even when the drive was fully trimmed up. Rotate the sender to the top, and same thing would happen when trimming down. Great...
Then, when looking at the sender I removed, I saw a different part number, on a label shrunk to the wire sheath: 21683581
Searching online for this part number indicated it would be for an EVC engine, which I don't have.
It also seemed to be discontinued. Great...
I did find a new old stock one on ebay luckily for less than half the cost of the other one. Maybe I should order another as a spare.
After receiving it, it looked identical to the 21484383 sender, except with a grey harness protector (instead of black). But it did not have stops when rotating the plastic prong, unlike the other sender. The 21484383 sender is analog, the one for my boat is obviously digital.
So check that first before ordering!
The next weekend, I tore into everything again and installed the new sender. Took a qtr of the time (of course), and now everything works properly.
And that's that.