Good luck on your research. I would enjoy reading the final product.
We have taken several cruises, and learned after the first that we preferred doing our own shore excursions. Exceptions could be for short (half day) stops and specialty trips.
Our experience includes Mexico (Cabo, Puerto Vallarta, etc.), the Panama Canal, Miami-Santiago, and western Mediterranean (Canary Islands, etc.)
We have often been with another couple - I might add having 4 is really ideal for savings. We were a larger group (8) on one cruise, and could most often get a mini-van. (The one time we had to use two cars proved to be the most negative experience as it was difficult to communicate with the other driver.)
In our experience it seemed to be best to be prepared to leave the ship as soon as departures were allowed, in order to give more choice of taxis. We always ask for an English speaking driver, and test this out by asking a few out of the ordinary questions. We learned that nearly all of the drivers have a good command of English for the usual sales pitch, but may not do so well in answering questions.
A caveat - there is often a manager close by to do the selling, and then you will be directed to the car - and the driver. If this is not satisfactory, be sure to refuse.
In some places, passengers may prefer to wait until later, after many people have left - you can probably negotiate a better price - after all, you may be the only customer left for that day. However, the English speakers may all be gone by then.
I always study the cruise line shore excursion list, and may even make a written list of highlights we wish to see. We have had good experiences showing this to the potential driver. We could often combine highlights of two or more excursions, minus the shopping stops. Drivers have often taken us to good out of the way restaurants - we buy lunch for the driver, if there are several of us.
We have never had a problem with breaking down, returning late, etc. But we do pay attention, and make sure there is enough time. I would be wary of straying too far. On a 2 day stay in Santiago at the end of the cruise we hired the same driver both days - including a day long trip up to the mountains - no cruise departure to be concerned about.
In addition to the savings, we can see what interests us, go at our own pace, stop for photos, have a visit with a local, and, in most cases, return to the ship for a late lunch and leisurely afternoon. On the other hand, if there is a shore excursion of particular interest, and if you are concerned about the time factor, by all means, take a cruise line excursion. If you are not part of a group it is usually not difficult to find other passengers interested in sharing - you just have to be willing to give and take on what to see.
We have always paid in dollars, and at the end of the trip. Yes, we do take cash from the U.S. - leaving extra in the safe in the cabin. We have not made reservations in advance. I would be glad to answer other questions.