Bakblade
For diy I found the bakblade on Amazon is amazingly easy. It’s a 5 inch wide razor on a 2 foot long handle. It takes me less than 10 minutes to shave my whole back, and believe me it’s horrible. I have only in my life seen maybe one or two men in person who are hairier than me.
Anyway, I use it once every month and a half-ish and way less in the winter. Sometimes its not bad being hairy lol.
The blade has little comb teeth just in front of it to keep the blade from cutting you up. If you have backne though it will find it, but I have never had an ingrown hair or irritated looking sandpaper-like stubble from it.
At 41 it has truly changed my grooming life. I think back to the years of my ex-wife unenthusiastically trimming my back with my electric Wahl, shaving with razors, applying Nair or myself doing all the above and ending up with a kinked neck or shoulder injury.
There is a version 1 and version 2. I have the v1 and it’s fantastic. The v2 may be better, but I have no reason to find out. The blade lasts 2 uses. After the second use I keep it around for other smaller areas (pits, thighs, rear cheeks, etc). Refills are available on Amazon for around $10 for 3. I bought a 6 pack of refills back in April and I have 3 unused and 1 on the handle that I used once the other day.
The are less expensive and similar items to the Bakblade. They’re garbage. And do yourself a favor, do not try to reverse engineer the blades. Yes, it’s just 3 old-school safety razors pressed/melted between to pieces of plastic. Yes I took it part and put new blades in. Yes, I filleted myself.
Good luck.