Sounds like the cut off you set is the same as mine. I didn't think anything in the box (racemic epi, IM epi, Benadryl, or Dex) would been helpful and from what I've read that was the case and I didn't know enough about the physiology to be sure. I think it's just such a rare thing that I am trying to brush up on the finer points of these airways, it's been a minute since I have came across this. The other problem is one I've mentioned in the past and not having a great sedatives, which is why I am trying to get a better gauge on how quickly these will progress and my limits on just how far I can stretch it. Otherwise it's go a bit rogue with a quick hit of fentanyl/versed and say sorry, not sorry after it's all said and done.
The way I see it, you only have two options in angioedema: You can intubate early before swelling becomes severe, or you can (possibly have to) cut the neck later. Intubating early is not necessarily ideal because from what I understand, even though the mild cases are anxiety-inducing, few cases of angioedema progress to extremis. OTOH, waiting has obvious disadvantages - if they do start to progress quickly you can be caught with your pants down. I think all things considered, keeping in mind how rare these cases are and how bad they can become, intubating early is probably the best approach, in general. Not necessarily in the field, but maybe.
Either way, once they are swollen to the point that they can't swallow their own oral secretions - or worse yet, have progressed to complete obstruction - they will almost certainly require a surgical airway. This is because at that point visualizing the glottis will be extremely difficult if not impossible, and since they'll have little physiologic reserve, you don't have time to spend trying things that are unlikely to work.
Because of that, I would probably not attempt to intubate this patient in the field under any circumstances. I would have done exactly what you did: oxygen, reassurance, as-rapid-as-is-safely-possible transport to an ED and give them as much heads up as possible so they can get all hands on deck. Have your airway stuff out and be mentally prepared to cric if things go downhill.
In the hospital, my approach would really depend on how well they were maintaining and how much / what kind of help I had. Once they start to crash, I would go straight to a surgical airway, just like I would in the ambulance. If I felt I had a little bit of time, a sitting, awake fiberoptic intubation may be an option, though I am not personally experienced with that. Maybe an awake attempt with the glide scope, but that seems like a long shot, especially if they can't lay back at all. A retrograde might be worth considering but I'd be concerned with causing coughing and gagging, which may worsen the picture.
All in all, it sounds like you did a good job. I probably wouldn't have done any different.