Unless you are coming from another sport with experience at interval training, I think focus on breathing is a 2nd order detail that should come after you get comfortable with the basic muscular feeling. I’d recommend initially using any breathing that just comes natural to you. Getting comfortable doing intervals (i.e. “getting comfortable being UNcomfortable”) takes time.
Personally, I would pick workouts that have short intervals and build to more of them before doing long intervals. The reason is that it is easy to develop bad muscular habits that are harder to correct (e.g. hip rotation, upper torso movement) as you get tired during long intervals (i.e. become uncomfortable). Once you develop good body position, pedaling technique and control, even as you tire, you can then expand your focus to more advanced features from timing of your breathing to practicing your pedal power in all parts of the circle, to higher cadence drills (i.e. without bouncing on the saddle), etc.
I think this is solid advice. I naturally do nose only breathing at lower efforts been doing that for a long time. Once I go over that line I try to focus on in nose out mouth as the effort gets harder. I agree on the bouncing at higher cadence that’s still a problem I am working on once I go over about 90. Earlier in my life from childhood I was very active through about 26. Fell off the wagon, but jumping back on. Pedaling is still a focus for me I have a lot more efficiency to gain there I think I suck at using the pull of my leg consistently if I am not actively thinking about it.
As in all things it depends… on your progression level and the type of SS work you’ve been doing.
If I’m doing a SS that is pushing up my progression level, it will usually feel quite hard but almost never push into the self doubt that Threshold does for me. Hard but completable.
If I’m doing a SS that is achievable or very similar to recent efforts, it will feel less hard and I will almost never have the “can I finish this” thought. Sorta-hard and achievable.
I like this 2nd type because it feels like work, and is mentally easier.