I use a bike trailer extensively around the farm and hands down the absolute best trailer is the
https://www.bikesatwork.com/. 300lb capacity!!! In fact, out of all the tools I have ever owned (everything), the bikesatwork trailer has been the best tool of them all. I have gotten so much use out of it without any problems, and it has saved me so much time and effort, and it has enabled me to move things around that I otherwise wouldn't have been able to. I can confidently say that buying that trailer was the best money I ever spent.
As for ease of pulling, especially on hills, what you really need is a high tooth count gear on the rear wheel, like 40 or more teeth. A standard mountain bike rear wheel has a replaceable gear cluster, so you can count the number of gears on your rear wheel, then shop around to find a cluster that has the same number of gears but with a much larger (more teeth) sprocket for the biggest gear. Just a few additional teeth can make a huge difference, so try to get the largest toothed one you can get that will fit your rear wheel. Any bike shop should be able to work with you on this to order the gear cluster and swap it out.
If you want to DIY it on the cheap, I would find a 50 tooth front crank sprocket and weld it onto a single speed freewheel sprocket on the rear wheel of a 26" cruiser bike, then put that wheel on your mountain bike.