In my VO2 workouts (in and out of TR) I’m struggling with longer intervals (60s and up) but the 30s ones from Goddard (for example) are easy-ish to complete.
In an effort to improve my VO2 weaknesses I’m doing more workouts in this range. Since the longer ones are currently v hard, should I focus on doing more 30s intervals per workout or make 90s intervals a bit easier so I can complete a full workout?
For the longer intervals adjust the difficulty using the in-app adjustment so that you can just complete the last interval. It’ll take a while to figure out exactly how much to knock the workload down by. Use the workout notes to record what you do.
Once you’ve found your level then work at increasing it 1% at a time until you are back at 100%
Sounds like you are trying to do two completely different things:
Improving your VO2max is different than completing longer intervals.
If you use a heart rate monitor, examine your workouts — both long and short intervals — to see how much ‘time in zone’ (>90% HRmax) you’re getting from each variation.
If lowering the intensity of longer intervals doesn’t deliver you into VO2max territory, then it’s not improving your VO2max (vs the short intervals) even though you’re completing the workout.
It’s fine to want both things, but don’t confuse the two. And don’t let your ego (long intervals) get in the way of your goal (improving VO2max). 
As @Captain_Doughnutman says, you need to stay in the VO2 max zone. It may be that there isn’t a big gap between your FTP and VO2 max levels, not everyone has VO2 max @ 120% of FTP.
I would say keep the intensity up otherwise they will just become threshold efforts. Start with Baird or Baird-1, then move on to Bluebell - both of these are 1 min efforts. Then you can move on to Mills. 2 mins descending from 120-110%. They are at the easier end of VO2 max. TBH if you can’t complete Baird or Baird -1 then I would say that your FTP is set to high as they are not that challenging.
I think 30 seconds and below probably don’t provoke the adaption you want. My guess is that 1 minute intervals do. That’s the theory I’m operating on right now, we’ll see. They certainly require more recovery for me than <30 second intervals. I see 15-30 second intervals as something analogous to strength training.
What i understand from the people smarter than me (i.e. the exercise physiologists :)) is that Vo2max is useful because it is a measurable proxy for the amount of energy you create aerobically (which cannot easily be directly measured) and isolate it from other processes. If oxygen usage is higher, then all else equal, more energy is being made aerobically.
The upshot of this is that any aerobic training, as long as it’s enough to stress your body into adaptation, increases Vo2max: threshold intervals do, tempo does, even LSD does. That said, from what i remember the research has shown that intense aerobic intervals drive more of these adaptations, but require more recovery (of course).
So if this is true, then i’m not entirely sure why short and intense intervals are the ones traditionally called “Vo2max.” One guess is that they are called that becuase they are at a power that will eventually elicit vo2max, and it’s in that case really more a description of the power rather than a claim that it’s the exclusive way to get the adaptation.
another reason might be the specificity principle, i.e. that you train at that power and duration to raise that power and extend that duration. It would be a bit of a misnomer though because i believe a (relatively) substantial portion of your e.g. max 3 min power comes from anaerobic contribution, if you come into it fresh and rest fully between efforts. And again it suggests that it’s the only way to raise Vo2max whereas really it’s not. According to Sebastian Weber it’s not even necessarily teh best way (he says that longer threshold intervals actually have more of an impact)
so, long story short there are a lot of ways to skin this cat. Very short intervals can raise vo2max if you do them tabata-style, i.e., with very short rests. At first anaerobic will be contributing to your efforts, but as you go through the short interval sessions it will eventually tap out and the rest will be intense aerobic.
Longer (but still short) intervals can also work to do the same thing. My view is you go for the highest power where you can still complete the session. But if you rest a lot between these intervals you will be getting anerobic contribution as well, and you’ll need to burn that out before you start accumulating the intensive aerobic work you need.
But remember you might need to be fresher coming into these sessions. i.e. to get the power high enough to justify the short duration.