It seems like what you are doing is good and seems to reflect what Greg is sharing in his videos, perhaps the endurance needs more time? I’m working on the moderato lessons, and one thing I do for the leadpipe is to hold my mouthpiece in one hand (fingers) and the leadpipe in the other and start the BCH breath into the mouthpiece (no buzz) and bring the mouthpiece and leadpipe together. I frequently get the buzz/pitch like that can can do the scales and arpeggios (one octave) that way. I can then do the same breath into the mouthpiece and bring the trumpet and mouthpiece together and typically get low c. After doing that a few times I work on harmonic slurs, tonguing, etc. trying to maintain throat openess, relaxation, no tension. (though my abdominals are firm, but not kicking) After some time spent on windworks I play my other exercises (like Clarke) and orchestra music trying to keep the same (or similar) feelings as the windworks.