A couple of things:
1) Your partner's serve is such that your competition can hit returns whereever they choose, meaning that if they decide to hit into your alley, they can.
2) Given #1, your idea to signal and sometimes poach, is a fantastic idea. The reason is that it ends up being very distracting and needlessly confusing to your competition such that they are playing very poorly and outright missing returns all over the place. My guess is your competition is not routinely dealing with the whole signalling/poaching thing all that much.
3) Aside from #2, your starting position when the serve is struck is deviated towards the middle and way back on the service line. True, because the serve is traveling so slowly that you have plenty of time to move across and forwards to poach or slide towards your alley, which is closer to the center of your responsibility. The fact that you are making this extra movement in the lag time when the serve is traveling over the net, is also maximizing the distraction factor for your competition.
4) As your partner gets pace on his serve and as you play competition who is less distractable, you will need to play more standard double positioning (lining up for the point closer to where you want to end up, ie the center of possible returns and then move from there for a poach), otherwise better competition is just going to make the decision to go into your alley before the serve is struck and is not going to second guess themselves with your movement and just burn you DTL. In addition if your partner gets pace on the serve you will be late to where you are trying to end up, so you will have to line up closer to the standard starting point (which is not where you are starting right now).