Great tutorial Eunan
I am not the tier you are but that's how I've been doing soft hackles when I had similar problems described in avs' post. Really stripping the feather and tying in at the tip were the first two things that came to mind.
Couple of other things are that I actually prefer my biots tied in UNDER the hackle. I just think it makes for a cleaner looking fly. The other is that I looked at Charlie's page and it looks like he's using a feather with a stiffer barb? Not sure what kind of feather you're using but India Hen is pretty easy to work with, though maybe not quite as durable as other feathers. Starling is really nice too. I bring this up because I tried a few a while back with stiffer feathers and I just didn't like the look of it and now prefer something with a fuller softer barb.
Sounds like you're diving deep into tying avs, very cool to see your progression. There are a ton of great tiers here who inspire me to learn and tie something new, Eunan being one of them.