Hi, good question and thanks for posting. Without actually knowing your current standings all I can do is offer some generalized info.
At any given time, the main deciding factor is the amount of your current standings. Remember to always check your Faction standings while working the Event Agents.
Now when first starting, you’ll want to maintain the Factions above -5.00 standing for ease of travel in Empire space. As you work the Event Agents and move up in levels, that base amount will change. Meaning after completing all of the level 1 Event Agents for all Factions, that amount will become +1.00 Faction standing in order to have access to all Factions level 2 Event Agents. When you start working the level 3 Event Agents, you’ll want to maintain a base amount of +3.00 standing with the main Empire Factions. +5.00 standing would be the base amount of the main Empire Factions for access to the level 4 Event Agents.
As you’re working the Event Agents, you may have to stop half way through one Factions set of Event Agents and ‘Bounce’ over to another Faction to maintain access. The Faction standings will do a ‘See-Saw’ while working ‘The Plan’. Just keep watch on the standings and bounce between the Factions whenever needed until all of the Event Agent missions for that level have been completed.
So you stated you’ve completed all 15 Gallente Career Agents. Let’s say for example your Caldari Faction standing has now dropped close to -5.00 standing. In order to maintain travel in Empire space, you’ll want to ‘Bounce’ over to Caldari and complete their Career Agents.
However let’s say your Caldari Faction is actually just above -2.00 standing, since it’s still way above -5.00 standing you can continue working the level 1 Gallente Event Agents and still have access to Caldari space and their level 1 Event Agents afterwards.
Eventually when the four main Empire Factions are close to having equal standings, you can then just do a ‘Round Robin’ cycle, alternating through each opposing Faction. For example first Amarr then Minmatar, followed by Caldari then Gallente, etc, etc.
I hope that helps, good luck and may you have much success.