Flash Elite C bootloader on Helix Keyboard

I've been running a Helix keyboard as my daily driver for the last year and have been loving it so far, unfortunately for the last couple of months I've had a bootloader issue that mean't I could no longer flash the firmware and customise the keymap. A lot of the guidance wasn't super useful on this but I managed to stumble my way into a solution which may help others in the same situation.

For reference I'm using Elite-C v3 on the helix board purchased from mechboards.co.uk around april of 2020

I would get the following error message when trying to run

bash -> qmk flash
...
Bootloader Version: 0x00 (0)
Erasing flash...  Success
Checking memory from 0x0 to 0x6FFF...  Not blank at 0x1.
0%                            100%  Programming 0x5F00 bytes...
[>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>]  Success
0%                            100%  Reading 0x7000 bytes...
[>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>]  Success
Validating...  ERROR
24231 invalid bytes in program region, 4335 outside region.
FAIL
Memory did not validate. Did you erase?

Most posts in github issues and stack overflow suggested that the bootloader was corrupted and would need to be reflashed, I found this guide https://beta.docs.qmk.fm/using-qmk/guides/keyboard-building/isp_flashing_guide on QMK which was moderately helpful but didn't work for me as I used a arduino UNO as an isp flash tool and this didn't show up in qmk toolbox.

This did help me figure out some keywords I needed though, I also noticed the acronym ICSP was on my helix board with some bare bins for soldering a header to.

I found the pinout of the ICSP header using a multimeter and the pinout I found on Deskthority. The right side of the board had the following ICSP pinout tested using a multimeter.

RIGHT SIDE OF HELIX, WHEN VIEWED FROM THE FRONT

MISO SCLK RESET
VCC MOSI GROUND

I could then use an ISP flashing guide to find the matching pins on an arduino UNO and burn a new bootloader onto the board.

Something to additionally note is that you may have to change your qmk firmware to use caterina instead of dfu-programmer. In rules.mk

#BOOTLOADER = dfu-programmer
BOOTLOADER = caterina

Then flash your keyboard again using qmk flash and you should be back up and running once again