1) USB3.1+ ports and drives should have no trouble with speed. I am not personally familiar with the Samsung T5 so I'm not sure what speed its SSD runs at.
2) Definitely update the SSD's firmware before using it as a boot drive. If Samsung has a macOS for updating the firmware, then it probably won't be able to update the firmware while you are booted to the SSD. If you upgrade the firmware later you need to make sure to have a good backup in case something goes wrong.
3) You have several options available. You could perform a clean install and migrate everything from the internal drive or from a backup. Or you can use an app such as Carbon Copy Cloner to clone the internal drive to the external drive. Sometimes cloning works better if you have proprietary software licenses to deal with otherwise all options should be equivalent.
4) If TRIM works then use it. Otherwise you should uncheck "Put hard drives to sleep when possible" in the Energy Saver System Preferences. This will allow the SSD to stay powered on during off times so that the SSD's internal management routines have time to work on optimizing the SSD when not in use. This might also be useful if the SSD has issues waking from sleep (some have this issue).
**** Make sure to always have good verified working backups all the time. It is impossible to recover accidentally deleted data from an SSD. Plus an SSD can fail at any time without any warning signs.
