I don't want to spoil the fun, but I already did this with a Pixel 6 (including newer ones) and my copy of UFED 4PC a while ago.
If you have full filesystem access either through exploitation or the OS being rooted, then they obviously have access the full filesystem, just as before. They can get WhatsApp, Signal and all the other apps' data. A physical extraction isn't useful because all data is encrypted by default, so extracting all the unallocated space in addition to the full filesystem would be a waste of time since there is nothing to recover there. In FBE each file has its own keys.
Cellebrite Premium is known to have FFS for unlocked GrapheneOS devices. They have for years, but they can't do any locked device extractions or exploitation of it. It's almost certain it's to do with exploitation of ADB attack surface. It isn't a useful hole for us to close as they'd still be able to open apps, change settings and browse without it, but it would be nice to understand how Cellebrite Premium's FFS is being performed.
As for caching, I confirm caches of deleted files could exist if the caches are not erased. It's conditional on what apps you use and how they deal with cache management. Use the "Clear cache" function in the Settings app for the apps and it will clear it out.
Certain data types configurable in the UFED selection screen can be turned on, but won't do much, like locations. This is because of lack of Play Services, Google Maps apps etc. It is still possible to get some locations by using cached geofence data of nearby networks (WiFi, cellular) - most of what would be cached are the locations at point of seizure though, or places the party would likely know where their target is already, like their home address.
The only things forensics tools do not cover are other data at rest e.g. Profiles. They would be able to know the apps installed on other profiles, but not their data.