This is a tricky one because basically 6 elbow strikes is a very strange thing to want to do anyway. Unless you're attacking with the triceps side of the elbow - say to the temple after a grasping block with your other hand has pulled them open and towards you, or down on the collar bone - an elbow making solid contact should be driving them backwards into a punching or close kicking distance, but you say at the end of the 6 elbows you expect the defender to still be close enough to grab by the neck then knee? Also, if they block one elbow they're very unlikely to be vulnerable to 5 more the same - far more practice to mix it up.
I'd suggest at the very least have the attacker vary the types of elbow - some coming upwards from next to the hip towards the solar plexus or jaw - which can sometimes reach between a defenders arms, some with your arm travelling horizontally towards their chest or face - often ends up clashing directly into their raised forearms, others "hooking" towards the temple with the forearm side of the elbow. They're less effective in general, but for completeness other options include a backward elbow after sliding the shoulder towards the defender and turning your back on them ever so slightly, which can itself lead into a spinning elbow if you don't think it's too slow and ineffectual to bother with.
It may be best to let the attacker try to create and then penetrate gaps in the defence dynamically (but not so viciously that the defender lacks time to get their pads in place before being hit), instead of picking the exact sequence of elbows to use beforehand. Footwork should follow the needs to the hip movement needed to deliver each elbow powerfully "through" the target... it's a function of the distance and relative position of attacker and defender. Remember too that an elbow can be very strong but has a very short range/depth of motion through which it's strong... if it's done ever-so-slightly too close or far away or at the wrong angle it just won't generate a usefully strong impact, so the attacker's distancing and footwork must match the defender's very carefully.
You say you're using muay thai pads and in my limited exposure their practice often ignores a lot of this... just elbow one side, elbow the other, one step forwards each time, but that's learning to strike a pad not learning to penetrate a guard position. If you're concerned about grabbing necks, "applying pressure" and leading into knees then it sounds like you want something integrated more of defender-aware focus - something building sparring/fighting tactical ability, and not just a moving punching bag - hence the details in my answer.