Gred Gross

As Benedict mentioned, a major downside to tankless water heaters, in my eyes, is that they need annual maintenance. Mostly descaling, which isn't a big job, but should be done. Kinda like changing the oil in your car. I've actually suggested to some younger folks that it would be a good marketable service to offer. That and checking on folks septic tanks. Surprising how many folks don't think about home maintenance items often enough.
If your tankless was installed best practice, it should have bypass valves to make descaling easier. You also should be cleaning the burner.

I hate to see items posted from Amazon, and worse, Temu. (Y'all know why...)
Go to your local farm store, or a good hardware store and you'll find some. Lacking that try arborist supply outlets, on-line.

The item you linked from the zoo is not a canner, just a pressure cooker. Besides being too smaller jars, one of the reviews notes that it does not make enough pressure for canning either.
That being said, some of my old time local neighbors do water bath canning for meats. Yes it takes hours but you can put a lot of cans in a big tub and boil it outside.

Since reading "Sproutlands" about coppicing culture, I've been looking at the power cuts in my neighborhood. They are filled with coppiced wood! Lots of alders in one place, poplars, oaks, maple. In the olden days, coppiced wood was popular because it doesn't take a big saw (or chain saw) to cut it. As firewood it doesn't have to be split, a big advantage. I'm looking at some of these poles for tool handles and other crafty things.
The book on coppicing looks good! Put me in the drawing!

Black locust will throw root sprouts a long way. I would cut it at the base. Coppiced black locust wouldn't be good for fenceposts (the heartwood is the resistant part) but good for other things.

Draining the hose is the best bet. Maybe you could put a barrel or something to hold a lot of water in the hoop house so you wouldn't have to use the hose as often.

I cook them like green beans. Braise them with a little olive oil and salt. Yummy and quick.

We use them for rock bags to hold down silage tarps and ground cover cloth. They last a few years that way.

I've made some biochar in my wood stove with nesting cans. Pretty small amount like you say, though.
I've also done the thing with steel drums, several ways. But a couple years ago I was determined to make a Kon-Tiki burner (see Ithaka Institute for the design and theory of operation), basically a big cone. I was lucky because my welder friend had just gotten a discarded fertilizer hopper (from a tractor implement). We closed the bottom and put three legs on it. It works great. I mainly burn brush of which we have plenty of, feeding fuel to it for 3-4 hours, and get about 5-6 buckets of char per use. You can also dig a conical fire pit and burn in it. I always keep a bucket of char in my woodshed to pee in. I just inoculated several buckets with fish emulsion as an experiment. And I throw it my my compost, and feed some to my cows.