Those are still popular in factories. maintenance can put tools is the basket and get to the broke machine quickly.

they're still available, in electric even these days! Though tbh you'll also get by perfectly fine with a normal bike that has a basket on the back, yeah you'll have to lift it up and it'll be a little bit wiggly with the weight up high, but it's absolutely fine if you're in normal health.

Funny, for me it was old men riding these tricycles around town.

You can still buy those.

Yes... I'd love to make one myself. I have an idea for one with swappable wheels: small wheels which don't stick up above the trailer bed (for carrying wide things), and large wheels for a less bumpy journey.

The hand cart which I have at the front is great for carrying large/wide things, but the wheels can't handle cattle grids or rabbit holes...

You're right, there are not many hills here. Only 1 small one I had to climb. I do think that with low enough gears and enough time, hills won't be much of a problem.

You could also do the hill in multiple trips, temporarily leaving one of the trailers behind.