Check with your local planning department. With 10" ducting, you definitely need a roof jack that doesn't constrict air flow. The standard Vent-A-Hood recommended roof jack is fairly low profile. It seems unlikely that it wouldn't comply with your local wind resistance standards. Are you sure it's not just your roofer's resistance to using something different from what he usually installs? It's a pretty sure bet that yours isn't the only house in Miami-Dade County with a 900 cfm Vent-A-Hood.

Is your appliance store recommending anyone? A local appliance store that provides good service should be willing to recommend someone who has done good work for the store's customers. Our appliance store recommended an appliance installer when we needed a modification to our hood and its venting. When the roof cap needed to be changed, the installer felt a roofer should do that job given our roofing type and what needed to be done so he recommended someone. That may be better than separately choosing a roofer and HVAC person. We got a pair that were use to working together and the job went very smoothly. I think the roofer charged us about $150 so it was pretty reasonable. About a week ago, we had high wind rain storm and no leaks so he did a good job.

There are solar powered fans to consider too. On a home with a gable vent at each end??? perhaps the "self-flashed" type that is installed very near your ridge (on the sunny side to gather solar in the summer) and would draw air from the gable ends. Another type fits in the gable vent with a wire and remotely mounted solar panel. I think you need to spend $500 to get a good unit but they are simple to install... like putting in any roof top vent, perhaps a $250 job contracted out? Easier, cheaper and as effective as retro fitting a ridge I'd think. Later you could add the soffit vents which should help draw cool air up to feed your gable vents and fan outlet. For a retro fit perhaps putting 4" X 12" louvered vents in every second void between your rafters would be a good compromise of effort and effectiveness. (In some older homes with no soffit vents there may be blocking between the rafters so check to see if the soffit air can get to the attic before you do all the work.) Radiant barrier foil: Perhaps there's a twofer here, especially if it's difficult to add the soffit vents for some reason. The stuff comes in 4' rolls and is inexpensive..... $70?? for 500 sf. Since it may not be easy to install it completely, I'm thinking, first to install it on the south and west sides on the underside of the rafters, thus blocking most of the radiant heat from the hottest part of the roof or gable. Installing soffit vents on the cooler side should provide a cooling draft across the insulation layer to feed our foil chimney. I suppose a full radiant barrier and full set of soffit vents would do better but perhaps the above would give 80% of the benefit for about half the work? (I'm lazy)

For the wall vent option, also consider the proximity of operable windows. Codes vary by locality, so you should probably call you local building inspector for clarification, but in my city in Northern California the vent must be >=3' from any operable windows. Folks above are correct that 90s in ducts result in pressure drop, but you're talking about a short run with a single bend, and you could always run a slightly larger duct if you were concerned about it, so I don't think the pressure drop from the single turn is a significant consideration because it's gonna work well either way. Primary considerations are: cost to install, how it looks, if smoke will find it's way back into house or otherwise annoy outside.