I have seen some videos on people using old EV car batteries, maybe 10 years old with still 80% of life in them, as batteries in solar banks. I would think that it makes good sense to buy used, cheap, but still good batteries, and put them in a battery bank for your solar setup.

I remember buying into the Ryobi 18v One+ line of tools almost 20 years ago because they promised to stay with their stem battery platform format. At the time, Ni-Cad batteries were the state of the art, but not very good. They lasted maybe 2-3 years. I thought to myself, if Ryobi stays true to their promise on keeping the battery format compatible, no doubt a new battery technology will come along that will be better than Ni-Cad's. Sure enough, the Li-ion batteries are now the standard. They are more powerful, last many years longer, and are less expensive the original Ni-Cad batteries I got with my first Ryobi tool kit. My 20 year old Ryobi Ni-Cad era tools work even better, and longer, with my new Li-ion batteries that cost less than I paid 20 years ago for the Ni-Cad batteries. That's being frugal!
I don't know if I will ever get into an EV in my lifetime, but I have replaced almost all my gas yard equipment with batteries. No more fouled up carburetors than won't run right or even start. Just slap a charged battery in them and I'm good to go. I used to buy gas push lawn mowers and grass trimmers every 2-3 years because the carbs died on me. I am happy to report that my battery grass trimmers are now about 10 years old, and my battery mower is 8 years old, and they all work as good as new. I have saved lots of money by going to battery power instead of gas in those tools.
Also, for my homeowner needs, my ~5 year old Ryobi battery chainsaws have never let me down whereas my gas Stihl chainsaw might not start the one or two times per year I actually needed it. Over the years, I have purchased a number of batteries for my Ryobi tools and run time on batteries is no longer the limiting factor for most of my jobs. I just slap in a fresh battery when I exhaust one.
The only trend I don't like seeing is that battery platforms seem to be going up in price at a much faster rate than older gas-powered equipment. When I purchased my battery lawnmower, it was just a little more expensive than a similar gas-powered mower, But now, it seems like a battery tool is 2X-3X more expensive.