Our exhaust slopes down form the high point for about 3 ft to a foot above the waterline, at the aft end of the topsides.
I like your idea of a full bore ball valve to shut it off. It would be at the start of the down slope to discharge, and readily accessible in our boat, and easy to install.
What is the consequence of forgetting to open the valve when starting the engine?
- Just failure to start?
- a big bang?
- Engine damage?

I know that you have NEVER forgotten, but perhaps know someone who has.

We have had water in the engine, due to a failed vacuum breaker. Now we have a pisser running full time.

For the new engine, we need a custom dry section of exhaust pipe, so I plan to make it rise at least a foot before turning down and adding water. That should be an additional protection.
I agree that the Vetus waterlock looks flimsy, but has worked fine for 2500 hrs and 9 years. Fitting anything else is a lot of extra work, so I will probably go with it.
We have an exhaust temp gauge and alarm, which will reduce chance of melting it. Our current exhaust runs at 27C (in Norway, I guess in the 50's in the Caribbean)