In ideal case you should use original charger of any phone or device. But it does not mean a charger can overcharge any phone or device. Once battery get fully charged it stopped the current and not charge over 100%.phone OS also check and stop power as per requirement.
After one UI 5.0 update Samsung dex is not working on my tv with HDMI cable it's working wirelessly but after connect to HDMI the Samsung dex or screen mirroring is not working
Dex is just interface to view mobile on monitor or laptop. You can connect 1tb ssd to samsung mobile and work in dex mode. I have not any 1tb ssd but I have tested 256gb works well.