Fitting this went remarkably smoothly (for me).
I will need to print a new case to replace the one I printed for the original board, apart from that, in Marlin mode, everything works fine.
In Touch screen mode, its getting some info from the main board, but for other things, it still thinks it's an Ender 3.
The touch screen config.ini file states
# In order the TFT firmware is able to provide all of its functionalities/features,
# ensure that the following options are enabled in Marlin firmware.
#
# General options:
# EEPROM_SETTINGS (in Configuration.h)
# BABYSTEPPING (in Configuration_adv.h)
# AUTO_REPORT_TEMPERATURES (in Configuration_adv.h)
# AUTO_REPORT_POSITION (in Configuration_adv.h)
# M115_GEOMETRY_REPORT (in Configuration_adv.h)
# M114_DETAIL (in Configuration_adv.h)
# REPORT_FAN_CHANGE (in Configuration_adv.h)
and
#### Machine Size / Build Area
# The TFT will auto-detect the machine size (min and max) in Marlin firmware (requires enabling `M115_GEOMETRY_REPORT`
# in Configuration_adv.h in Marlin firmware).
# Format: [size_min: X<minimum distance> Y<minimum distance> Z<minimum distance>]
# [size_max: X<maximum distance> Y<maximum distance> Z<maximum distance>]
# Unit: [distance in mm]
# Value range: [min: -2000, max: 2000]
Hmm, as I type this, I think I might know whats wrong, while M115_GEOMETRY_REPORT is enabled in Configuration_adv.h, the bed size is set is in Config.h only.
I always presumed it got all it's info about things like travel speed, bed size etc from the main board, looking at the touch screen config.ini file, it looks like I might have to specify a lot of it again.
Oh well. not a huge deal, at least I've not had any bed leveling problems in a year lol