bootloader and firmware update for vesc 4.12

Hi all, I have tried to get my head around this since last week by reading before committing to write on the forum...

I posses an old vesc 4.12 (this is what is written on the board), from 6 or 7 yeas ago and I want to go back to my eskate project. Unfortunately it seems very old of the new vesc tool.

With all the information a found and a bit of help from gemini, here is what I have done so far...

1) tried firmware update (from 2.18 to 6.05) from the vesc-tool with no luck (no reboot, and the I reconnect back to see version 2.18)

2) tried firmware update from 2.18 to 3.66 from the vesc tool (hoping a smaller gap would help), same as before

3) tried to update bootloader from vesc tool. it look like nothing happens. Everything seem to stay in the exact same state.

4) bought a st link (exported the state of memory before flashing anything) and flashed version 3.66 . Now the vesc is not recognised by vesctool at all.

5) brought back the ancient state of the memory with the stlink-gui tool (i am on ubuntu). Now the vesc is recognized again. 

6) tried the firmware update from vesctool again and.. well it seems something happened because not my vesc is not recognized by the vesctool anymore.

Now my next step I guess is to actually find the last version of bootloader to flash it with the s-tlink (gemini explained to me it may be my bootloader is to old and cannot make the new firmware boot)

My question then is where do I find the bootloader?

I searched on the git repo and can't seem to find anything related to firmware or bootloader. I did find the firmware somewhere but I can't find it back, I get a bit tangled with old repos and forks from blcd tool, and I assume those are not the latest version.
My vesc-tool is precompliled and I only have a binary file so I don't think I can find it on my filesystem...

Would anyone be able to confirm I am going in the right direction and help me find those files?

Thanks in advance !

Rosie