I think this might be a case where downsizing is actually a bad idea.

You have a six year vehicle that serves it's purpose, why not invest in a micromobility option to save fuel while you're not towing your trailer?

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)

I have a 2WD 2019 F150 with the 5.0 I use to tow my horses around. With the tow package, I think its max is 12900lbs and it does a hell of a job on stuff in the 6-9k range but get up in the max range and it struggles a bit. I would say it gets about 8-12mpg depending on what we tow with it. I think it's rated for 19/21mpg just on its own. I get about 17/19mpg just driving it around.

It lives a light-duty life hauling horses down highways and South Texas caliche roads, but I am happy with my f150 most days. I think when it comes time to replace it I will go back to the f250 for the higher towing capability but stay gas only because everything we run is gas.

Hopefully, my experience helps, have fun truck hunting.

Diesels are really nice and generally get better mpg than gas when towing, but their maintenance is more expensive and they are significantly more expensive to purchase in the first place. They take more oil, more tires, and nowadays require DEF that partially negates the fuel cost savings. Plus a lot of modern diesels seem to have consistent emissions equipment problems.
They're amazing if you're moving very heavy loads all the time or travel over mountain passes frequently, but at only 5000lb payload I think a large diesel is overkill in any other situation.

Depends on what you need.

I use my truck about the same as you. But get 13mpg towing, and 18 not.

1995 Ford 250 PSD.

It's not comfortable, it's not fast, it's not fancy. But it'll pull whatever I put behind it. Carry almost anything I will ever need to (definitely more than I want to physically unload), and it's economical, and "future proof" in that I can modify in almost any way and still register without new diesel problems (high pressure pumps eating the entire system, def fluid and filter plugging, egr cooler plugging etc.)

Personally I'd say get an older diesel that is in ok shape that you can make what you need.

How hilly is your terrain, and how often do you pull the trailer?

Mostly flat and not pulling often = 1/2 ton, although you might occasionally be annoyed with getting going slowly with the trailer.

If hilly and/or pulling often going for the 3/4 for more consistency would be better.

It is like picking between economical hatchback. Being low powered isn't generally a big deal if you aren't loading the back with heavy stuff and driving around flat roads, but if you use the hatch to move a few hundred lbs a few times a week uphill then getting one with a more powerful engine makes sense even if the hatch size is the same.

The biggest advantage to diesel in my outdated experience was low speed torque for heavy loads, especially trailers.

Kinda dead, but there's a community for vehicle suggestions.

!whatcarshouldibuy@lemm.ee

Toyota. You're a fool to buy domestic, especially if you're only towing 2.5 tons.