I spent one or two days to find the original KSGER FW, just in case I need a chance to go back.
Now I can confirm that, for my KSGER device , the backup original ksger 3.1 fw can be flashed and it works again.
(my ksger: screen sysinfo HW 3.10 SW Ver3.1s , PCB black , silkscreen OLEDV3.0, Stm32F103RB 64pin , the tiny battery connector below the STM32 chip),
Using the FW from some page in this thread, file name --- 3.1Snew1.hex , it works, it can be loaded to the system, it didn't ask for some register code, and it has the same spell error and bugs or signature behaviour(option 5 code end (should be cold end) , Standby -- doesn't allow user to set zero and save (after system reboot it's default to 10min), sysinfo says the same version 3.10 and 3.1s, the only difference is the sys start info says WILDA , not Ksger.
(firmware attached --- 3.1Snew1.hex.zip )
For the flasher, a cheap and popular 2$ stlinkv2 clone works well.
So, hopefully more people with my device model or similar model can relax and encouraged to try the customized open source FW and knowing that they can always go back should they choose to, without worrying anything about losing the current solder station or desoldering a 64PIN chip to backup or getting a new same chip to replace or buying a new stm32 discovery dev board to do time glitch attack, or porting the original attacker program from asm to C and from a weird discovery STM32F303VCT6 (not so cheap, out of stock , and paying high shipping cost ) to a popular blue pill (stm32103C8T6), while the original attacker program is using PD port while the bluepill doesn't even have that port (PA PB PC only )....good to know that all those problems are gone now.
When flashing/erasing, need to solder a 4 pins header connector to the ksger pcb, CDGV, but the TH holes are a little too tiny, so just use a filler to work on the new pin header to make them a little smaller, then they can fit in. No need to remove the PCB from the front panel (mine is CNC Ksger panel, looks good, but not easy to remove the pcb )
Another note, when booting the Orig Ksger fw with stlink, need a twezzers to short the capacitor C4 below the stm, which is the reset pin - and Gnd, then it can be connected using STM32CUBEProg program.
then destroy the read open option byte and at the same time erase the KSGER protected fw.
And, thanks all contributors !