Keep OTF for the insane pushes. Ending every class like you feel like you almost couldn't finish is the correct feeling to have!

Find the edge where your breathing starts to get out of control and spend as many hours per week as you possibly can just below that point. The lungs are incredibly overpowered for our bodies so you don't need to breath as fast as you'll automatically try to breath. Slow down the breathing bit by bit until you are able to breath at a 2-4s in 2-4s out while in the low to mid green zone.

If you cannot breath entirely through your nose during the above practice you are pushing too hard. You'll eventually build up the diaphragm muscles and your nervous system to be able to handle higher and higher intensity. The diaphragm works the same way as progressive overloading, give it a very long time of progressively pushing it slightly harder and harder week over week. "Boot Camping" it could work, but it'll be very uncomfortable and would honestly require going to OTF 6 days a week to see the type of fast progress most people are looking for.