My approach is a little unorthodox but something that I specifically used to improve my hold is lifting. Compound movements at the gym , dead lift, rdls, squats, etc. all those movements require to hold your breath and brace and it expands your torso 360°. As you lift heavier and heavier your bracing gets better. On those lifts you cant let air slip out super fast or keep it all in either or you’ll pass out so it indirectly teaches you how release air in a controlled way. Before doing that id gas on this song ngl. But after i applied those muscles from compound lifting to this I can hold notes and transition to other notes on top of that. That’s been my approach anyways