If it’s straightforward, I would probably suggest the removal of the Etc timezones as well. With the exception of some POSIX / time related Discourse instances, these timezones will be largely irrelevant and often confusing if a user wants to target an offset rather than a locale. They would appear before GMT/UTC in the list and the average user won’t know what the Etc prefix means.
If not removed, the prefix should be included in the display of the inserted time. Perhaps even with some indication that it might not be what the user expects, e.g. Etc/GMT+1 (= GMT-1)