Good evening again Dan, thank you for your suggestion of using Playlist Creator.
I managed to create a bunch of .pls files! Yay!

However, only one of the playlists is showing. Not entirely sure why

I agree with the use of pls files, much easier to edit. I've opened them in Notepad and I can't really see any major differences in the file naming conventions between the two, e.g.:
Playlist 1: David Attenborough.pls:
[playlist]
File1=1979 - Life on Earth\Life on Earth - Episode 01 - The Infinite Variety [HD 720p].mkv
Title1= - -
Length1=3257
File2=1979 - Life on Earth\Life on Earth - Episode 02 - Building Bodies [HD 720p].mkv
Title2= - -
Length2=3225
File3=1979 - Life on Earth\Life on Earth - Episode 03 - The First Forests [HD 720p].mkv
Title3= - -
Length3=3200It continues for a while like this, then ends with:
File243=Others\2022 - BBC Dinosaurs - The Final Day [HD 720p].mp4
Title243=Dinosaurs: The Final Day with David Attenborough - - Dinosaurs: The Final Day with David Attenborough
Length243=5250
NumberOfEntries=243
Version=2This playlist works fine and play on all my devices.
However, this one: Brian Cox.pls:
[playlist]
File1=01. BBC Wonders of the Solar System\BBC Wonders of the Solar System - Episode 1 - Empire of the Sun [HD 720p].mkv
Title1= - -
Length1=3502
File2=01. BBC Wonders of the Solar System\BBC Wonders of the Solar System - Episode 2 - Order out of Chaos [HD 720p].mkv
Title2= - -
Length2=3516
File3=01. BBC Wonders of the Solar System\BBC Wonders of the Solar System - Episode 3 - The Thin Blue Line [HD 720p].mkv
Title3= - -
Length3=3539and ends with:
File39=10. Others\2017 - BBC Brian Cox - The 21st Century Race For Space [HD 1080p].mkv
Title39= - -
Length39=3537
File40=10. Others\2022 - BBC Brian Cox - Seven Days on Mars [HD 1080p].mkv
Title40= - -
Length40=5335
NumberOfEntries=40
Version=2...doesn't show at all in Serviio. Odd.
I've tried saving them both in the same folder, in their respective parent folders, and in a new folder called 'Playlists'. Sadly only the David Attenborough one ever shows in Serviio.
I tried messing with the settings for Playlist Creator and saving again and again to no avail. I can't seem to replicate my success with the David Attenborough one.
Good luck with the Revenge of the Nerds recording! It sounds laborious, (and yes, chopped for TV ad breaks

). I think I have a 1080p x265 copy of that somewhere on one of the HDDs...
Thanks again for all your help,
Schmaulie.