Him i have used office with Dex and it works well. I personally prefer a portable wired monitor for dex on my S10+ when using office apps as wireless Dex on my huge TV has text too small for my eyes.
But yes, Dex works well. It is pretty smooth and you can multitasking with it and you can still get texts and notifications on it. I use a wireless keyboard and mouse with mine.
Hope this helps.
I got the note 20 ultra, and I have a wired portable monitor showing up tomorrow. I will spend next week exclusively using just the note for all my work needs. I have a feeling its going to crush it and work exactly how I need it to work. I messed around a little bit on my son monitor with dex, I was actaully blow away by the phone having gestures like a track pad. 3 Finger swipe switched between open apps like it does an a pc.