Install TWRP and Magisk Manager on Moto G5 Plus without rooting
Answer: it is not possible without unlocking boot loader. 
Thank you to @alecxs !

Also for anyone interested, rooting has some side effects: cannot load the Netflix app due to safety net checks. Even with the magisk mod for hiding safety net flags. 
And ultimately, ARCore still doesn't work, even after I flagged the Pixel 2 "image", and installed Google play services for AR.