Ok it seems you shouldnt be losing threat in low level dungeons if you're doing as you say, and using STR accessories.
Something that helps direct the party, based on that example with the 2 NIN's that is in game is the marking system. A good macro would be:
/marking attack1 <t>
/ac "Shield Lob" <t>
Keep in mind that enmity generation has been increased for all tank classes at an absolute minimum of 3 times. If not more, I havent kept track, and players in the "vanilla" days were able to maintain threat.
I'm not saying adding an enmity bar that can be toggled is a bad idea, I'm saying its not very useful for the developers to add it this late in the game for specifically low level content, which you admitted would be its only real use for. They already adjusted enmity 3 times for this specific complaint.
I'm offering advice to you as a player, in hopes to help you maintain threat better. Not because I think you are bad, or I am better than you, but sometimes people arent aware of certain things that can cause a situation to go unexpectedly, and a more experienced player has seen how certain situations can do just that "go unexpectedly."
The best way to approach such things is to weed out possible more common errors.
My only other suggestion now is to try to physically watch the dps players. Meaning, visually watch the players character to see which enemies they are fighting.
If you use the "attack1" mark and they are not fighting it, you can ask them to either 1. use their enmity reduction action, 2. do as directed based on the attack1 marker or 3. vote kick them. Its considered "difference of playstyle" and not considered an offense to kick someone for not listening to the party. You also can just leave yourself. If something isnt going how you like, in a pug situation, no one is forcing you to remain there. The point of the game is to have fun, and if someone is purposefully doing things that arent helpful, you dont have to party with them.
2-3 flashes, 1-2 overpowers/unleashs should be plenty to maintain threat with the current enmity multipliers, followed single target on any job doing single targed on a mob. Split dps is split dps, and is a dps/party issue.