I have a homebrew rule for this, though I don't know how good it is. My players seem to like it:

If you wish to "hold your action", but do not want to use your entire action for it or interrupt another character's turn, you may instead *Hold Your Initiative*. This will allow you to take your action in a specific and later place in Initiative, but you will not be able to wait for a specific "trigger" that could interrupt someone's turn. However -- you will need to permanently take this new initiative for the rest of the battle.

Also, any positive effects that last "until your next turn" such as "shield," will end on the old initiative, while negative effects such as "disadvantage until your next turn" lasts until your new initiative. (So you cannot use this to extend positive effects or shorten negative effects.)

For example, if your initiative is 19, but you want to hold your turn until after someone has a chance to heal another player, you may Hold your Initiative until after the turn where it happens. The player is healed at Initiative 15, and you are able to take your full turn on initiative 14. From then on, your initiative is now 14.