Now I know this may not be the most popular idea but hear me out.
Why not have a Free Lock Faction, what this means is simply put you can just say "sorry boss I got a better offer across the street." in STO terms it means, both either the federation or kdf players can choose to resign from the factions they are in and take their ships with them, however it would force a decal on the ships they have that are faction based. this would have to be top level based.
this would also mean their Z-Store ships would stay on the previous faction, and they would need to remove all items from the Z-Store ships before they could begin crossing over, or they can dismiss the Z-Store ships they have.
anything not bought on the Z-Store can be transfered with them. so the Aniversary Oddys can go, any lockbox can go, as well as fleet ships and rank up reward/dil purchased ships.
and then once transfered over they would be able to buy romulan ships and reman ships.
this can be brought into lore by having it stand as emigration or Retirement. since all the ships obtained are the property of the admirals who obtained them it seems somewhat plausible.
it would also allow for people to start out as a romulan, but to quickly populate the romulan faction, the transfer could work.
but once this is done, the Admirals cannot turn back.
I had a bunch more things to add to this, to make it more apealing but my brain just stopped after 3 hours of writing this, between going back and forth with my baby niece and just plain housework it just stopped flowing.