In follow up to my own question. I think the lumpy lip slur is a result of not consistently finding the sweet spot in the positioning of the aperture corners within the teeth space opening and ensuring the bottom lip is rolled out enough (milk spout). This sweet spot when found allows a quick shape change snap between harmonics. If I can stay in the sweet spot the speed of the change of shape appears to be the same ascending and descending.
After revisiting all the Largo fundamental videos and looking at the shape of Greg’s embouchure “Mouthpiece revisited” @ 4:53min, I realised wasn’t doing the milk spout idea enough. So in experimenting I took the Oooo part to the extreme and really rolled out my bottom lip. This resulted in a surprising discovery. With the bottom lip out in front in the milk spout way, my top and bottom teeth aligned without effort/force and my top lip came down into the middle of the space between the teeth fully relaxed, exposed to the airstream, dangling freely, oscillating sympathetically. It felt super weird when introducing the mouthpiece due to rolled out bottom lip, but the sound was resonate, responsive and importantly I could feel and manipulate the aperture corners in a way I haven’t encountered before. This has resulted in the lip slur snap being crisp especially when I can find and maintain the sweet spot.