weighted triceps dips

I'd like suggestions for improving my weighted triceps dip. I'm
training to compete with a friend at the end of October - 5 reps at
the most weight on a dip belt

Good assistance exercises?

What's "good enough" depth?

Form and technique recommendations?

Breathing strategy?

Weekly schedule?

Stretching tips?

I've injured a shoulder in the past. 2 weeks into training and I'm
using 75 pounds for 5 reps. I did 90 pounds for 4 reps today. My
"bum" shoulder let me know it's waiting for me to make a mistake.

I'm 57, 220 pounds, 5'10"

Thanks for any help.

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