Okay so, having read this, I used Daybreak to install 18.0.0 and now Atmosphere doesn't boot at all. I get:
"UNKNOWN PKG VERSION
HOS VERSION NOT SUPPORTED
OR emuMMC CORRUPT
Trying Backup Bootloader
foung pg1 <'20240207110330'>
Unknown pg1 version
HOS version not supported
or emuMMC corrupt...