How does rooting affect full disk encryption (Oneplus 3T)?
no, rooting does not affect encryption   
encryption remains safe. depending on user (who is responsible for granting root access) it is not vulnerable as long as malicious app has no root permissions  
rooting does no factory reset. once rooted, the simplest straight forward backup method is (FDE)
adb pull /dev/block/dm-0  
rooting system is outdated. systemless root does it's stuff without modifying system (to preserve functionality)  


However, the answer becomes invalid when we are talking about unlocked bootloader  


attacker can flash modified boot or recovery  
TWRP may able to decrypt userdata partition without asking for credentials when FDE uses default_password (which seems the case for you)  
attacker can inject scripts without having access to encrypted userdata, which can wait for the user unlocks it's device and then gain access to decrypted userdata. for FBE it is possible to modify credentials (not the case for you)  
unlocking bootloader requires factory reset. your only chance for full backup would be a rooting method without unlocking bootloader, which is afaik only available for Mediatek devices and for the Galaxy S9 and Note 9. you rely on common (non-root) backup solutions