loose prop

To each his own. If the propellor is installed properly, and everything is correct, there is no situation that will cause the nuts to loosen on their own. Usually you need a 4' breaker bar to get the nuts loose, when you want to remove a prop.

What will happen sooner or later in your situation, is the shaft will snap off between the strut and the propellor and you'll need a shaft and a propellor. I've seen it happen several times over the years.

I have an owner who I've dealt with since 2008, with a 2001 50' Post with 820's that ground up his props in the Bahamas, the diver had a terrible time getting the nuts loose and props off.

This still isn't fixed... seems like a simple enough task...

You could always switch to a castle nut for the second nut. I've changed wheels for years and the only time I've seen one loose on haulout was when they hit something and continued to run it shaking like hell. I've used neverseize on the taper just to help it slide up a little easier. Every yard seems to have a theory on how to install wheels. Have you seen them tighten the wheels? There's a few things to check but it's not rocket surgery.

I didn't see any mention of it but,

Has the boat ever been aground and really banged the props up?

Do you get any strange oil leaks, loose parts , things falling off for no apparent reason?

Are you cutlass bearings wearing evenly?

On the other end;
How is your clutch coupler set up? Tapered shaft into the coupler or a blunt end captured?
After your incident 10 years ago and loose nuts for a while now, I wonder if something is not perfect on the other end and fine vibrations come down the shaft.
Usually if it's a tapered shaft coupler, it would have a single crown nut and a pin thru it or a self locking (NyNut) and a pin above it. I guess you could rotate ends next inspection.

Another thing to check is the length of the Propeller Hub, as I have seen some non-standard Prop Hubs that are shorter than the SAE requirement, and therefore do not provide the required contact length along the shaft taper.

What diameter is your shaft and what diameter is your propeller?

Since K1W1 and Karo1776 addressed how to use Prussian blue and how the taper should lock the prop on, I think you need to check the nuts and shaft threads. I would check the threads on the shaft and inside the nuts with a ASME thread pitch gauge (especially in region where the nuts are when they are tightened). Then with a dial indicator check for excessive backlash in both X & Y axis. All nuts on threads have backlash, your looking for more than ASME allows. The threads on the shaft could look not stripped or pulled but a improperly ground bit on a lathe can make great looking threads but out of tolerance.

According to Steve d'Antonio, you put the big nut on first and tighten it to the proper torque. You then remove it, install the small nut, and then the big not on top. Having said that, I don't go through all that. I lapped the shaft one time and now use a PropSmith to put the props on. I've had no trouble.