Hi@kpt-fish
I was a complete beginner but managed to get /e/ on my SGS3 i9300 from 4.3 more by not giving up than following the instructions which are not simple to follow. I tried but could not get Windows to work I have Windows 10. What finally put me onto Linux is there is no release of Heimdall for windows you have to build it. I also found the device dropped out using windows
Ignore anything to do with OEM unlock it was not loaded on the S3. I used an SD card to load TWRP twrp-3.3.1-0-i9300.img that worked for me.
Heimdall installed easily follwing the e doccumentation for our phone.
Install e from custom recovery, ie set the phone to recovery home, vol UP and power. Sideload as they suggest was the only way it worked. Load the file onto the linux computer, I used
e-0.9-n-2020051153544-dev-i9300.zip on the computer and side loaded on the second page of options on the device, do one then the other with the lead attached. Whatever you do before wiping, make a backup copy of the rom, I did not and nearly bricked the phone.
When you have it, updating is easy, put a copy of the new ROM on the SD card, use the backup and recovery from the device and choose the SD card, then restore, it works.
Good luck let me know if you get stuck.
I was worried about downloading a custom recovery from terminal emulator or adb shell but I found a superb website with line to line instructions, I’ve spent the last 48 hours trying to find it but can’t.