Any way to flash SuperSU without TWRP?

So far, every damned Cricket phone I've ever bought has either a crippled bootloader, or no bootloader at all. I recently bought the LG Harmony (M257) (Android 7.0) from Cricket Wireless after unsuccessfully trying to root and bricking a ZTE Grand X 3 (Z959) (Android 5.1.1) from losing charge while attempting a root. Either way, these phones seem to have little to no bootloader. If you prompt adb with "adb reboot bootloader" it just boots as normal, right into the OS.

My whole question here is, is it possible to flash SuperSU without TWRP or an unlocked bootloader? I am in dire need of rooting this device, as I'm getting overwhelmed with how little it can do as stock.

If there's a downloadable version of twrp that is "universal" somewhere, with a proper signature that can be adb side-loaded, or anything that can get me a team win custom recovery on a device with a crippled bootloader, please tell me about that too.

Thanks in advance to anyone that replies with helpful info!

- it has a bootloader; just crippled (like most carrier supplied prepaid devices)
- no way to install Magisk, SuperSU or similar tool on an unrooted device with locked bootloader (if it were that easy ...)

Well, it was worth a shot... Guess I might just have to resort to a few sketchy one-touch methods and use SuperSU Me.