Diesel is clearly better if you are driving 20,000+miles per year. However you are not doing near that, so it won't be worth the extra cost. Today diesel is so much more expensive than gas that the real advantage is only that diesel engines last longer, and in your case the body will fail first.

How much of your driving is towing vs unloaded? If you are only towing then a large engine is better - displacement = torque = more fuel efficient. However if you are mostly unloaded something like the Ford Ecoboost engine is much more fuel efficient unloaded and when towing you lean on the turbo to use more fuel (as much as the large displacement engine!) and so still have the power - but the engine won't last as long overall and will break more often - thus not a good choice if you mostly tow.

I would lean to the 3/4 ton trucks. While a 1/2 ton truck has the specs to do the job, all of them are aimed at the luxury car market these days, and so they will make compromises that make them not as good for real work. 3/4 ton still is targeted at people doing real work and so they will have better compromises. (if you were asking 30 years ago a 1/2 ton would be fine)

Do you need something now? Electric trucks are just coming out and should start hitting the used market soon. They only do about 100 miles when towing, but are much more environmentally friendly if you can live with that limitation. I wouldn't think about sticking with the truck you have now for 3 more years to see what happens here (and also 3 more years to get real world experience with how electric trucks really work for people in your application)