You should consider that just because someone may have feelings of lust for you, that does not mean that they would go so far as to leave their partner for you. Even if they like you intellectually and feel very strong passion for you, they may feel the same way for their partner, and even if they don’t, guilt and fear could prevent them from being disloyal, and perhaps even from breaking up.
I don’t have experience working with the Goetia for this sort of situation, but I will say that for this sort of working to be effective, timing is very, very important, as is information gathering. You need to know if your target can be swayed to disloyalty, or if an outright breakup is the better approach. You also need to know about the status of their relationship. I advise divination.
The former of these options can be tricky to pull off. The latter can be easier since people are often incredibly horny and perhaps even looking for revenge sex after a breakup, so you can position yourself as the “rebound,” which is also much less work than keeping an affair secret. If you go for the breakup, it is quite simple to cause, but requires patience. Arguments are a good way to start it, but they need to lead into resentment and disgust, not reconciliation. Causing sexual impotency in the other partner while increasing your target’s sexual desire is a good way to increase the strain and have the target start considering other options. Another approach is to cause the other partner to lose interest in the target, but not become impotent, and this is especially effective if you bring another sex-interest into his life to capture his attention. This is where the earlier work to cause passion and good regard will help you. It also helps if you are in good standing with her friends, whom she will likely talk to about her situation.
This clearly requires more magickal work than most would want to bother doing, and more patience than most would want to commit. In all honesty it would be much easier to just attract another partner if all you want is sex. Hot women are in abundance. If you hate the guy or just feel the need to destroy someone, though, then this can be quite the enjoyable working to navigate through.
As for your choice of demons, I would not call Asmodeus of the Goetia to cause a breakup. Gaap can conjure fierce hatred between two named targets, cause lust, and cause sleepiness and loss of concentration. Leraje can first cause disruption and arguments, to lead into the hatred, and if necessary can bring pain as a distraction. Dantalion can manipulate both thoughts and emotions, and can bring about meetings with potential new partners. Sitri would be more appropriate than Sallos here to ignite lust and give you a seductive aura. You can evoke Sallos or perhaps Berith later if you want to make this to lead into romance. Paimon can improve your reputation and help you with conversations with your target. Seer can bring resolution and swift decisions in your favor. (source - Demons of Magick)
Astrological events are mostly irrelevant with Goetia magick. They can support magick, but I honestly never pay much attention to astrology and my magick works just fine. I do my rituals while sitting on the ground and I often wear a hood, but the wording of your requests do actually matter. My approach is to state the desired end result as my request (or command to reality), while keeping in mind the specific powers and aspects of the situation that I want the individual demon to cover. With these demons I would give offerings as the situation develops and individual results manifest. It is also a good idea to tell the demon of the other rituals that you have or will perform, and to at all times be clear with the overall end result that you want. Do what you must do to achieve this, no more, and no less.