OK, have gone through the process and emerged on the other side. For the benefit of others who may end up following in these footsteps (eg @forcher), here is what I did. Bear in mind this was done on a Linux machine, so there are a couple of extra steps.
In the end there were 13 steps in two main phases:
Phase 1: prepping.
Phase 2: doing.
Each step often consists of multiple sub-steps, but these can be found in the documentation. The trick is to write your own install procedure with the steps (and their sub-steps) required. Doing this makes it run much more easily.
I can’t emphasise enough how important Phase 1 is. It consisted of 8 of the 13 steps. These were:
- Setting up a folder tree to house all the files
- Downloading Xperifirm (with the sub-phase of getting it to run on Linux by using
mono) - Getting the stock Android 9 ROM (I had to use the one from Brazil to get the right flavour, i.e., Pie!)
- Getting Newflasher
- Getting the /e/ ROM & recovery files (with SHA256 checksum)
- Getting the copy partitions file
- Getting the phone ready (copy files off etc)
- Setting up adb & fastboot
Then the doing:
- Unlocking bootloader (if required; I had already done it, but for new starters this is needed)
- Flashing the stock Android 9 with Newflasher
- fastbooting a custom recovery ROM
- Making partitions consistent
- Flashing eROM from recovery
So, how did it go? Pretty well. A couple of nerve-wracking moments after Newflashing the Stock ROM, but OK.
I’m now experimenting with the audio/mic 3.5mm jack. I’ve noticed some problems with the sound. Often choppy cutting-out sound for music, but also for phone calls. In fact, I can’t use the mic/earphone lead at all now for calls whereas I could for Oreo/8.1. Bluetooth sound seems to work OK, as does the normal speaker. I’m sure I saw someone else mention sound probs, so will go looking.
When I did this first time around 2.5 years ago, getting the Windows machine to even see see the phone was the issue. No problem under Linux, and adb/fastboot are easily got from repos.
Might need to re-flash it again and give it a few reboots under stock A9 with music to see if that fixes the mic/earphone problem, before moving on to /e/.
Again, many thanks to @chrisrg for the offline message tutorials. 
UPDATE: The above was for Q, not R. Has anyone tried R and, if so, was there any issue with the headphone/mic jack for sound?
I know the jack is working because it worked flawlessly when I tested it on Stock 9 during the second re-install procedure. Perfect sound. So, the issue seems to have arisen after that with the Q ROM. Still working on it… Maybe if I just go on to R…