Before, I was in the relationship with the couple that ended badly, I was in a relationship with another couple that lasted a couple of decades - until his passing. Like you, it was necessary to refrain from any PDA. Initially, it was tough for me. However, I ultimately decided that the outside world was not a good frame of reference for determining my secondary status, especially since inside the house, I was never treated as secondary. I was an equal partner. It sounds to me like your couple is doing what they can to integrate you into equal partner status. Are you willing to lose not only your lady love, but what sounds like a loving, considerate metamour due to no PDA?
Also, much to our relief and amusement, we would take vacations to places no one knew us and PDA to our heart's content - all three of us. It was fantastic to watch the reactions.