Re: thin neck
Neck Harness is your friend. Bridges are more likely to cause injury and you cant progress each week with more resistance, so for that reason I reccomend the neck harness in multiple directions.
This is the one time a boxing athlete should use body building methods, yes you want a strong neck but you also want it to be as thick and big as possible like Tysons and Holyfields.
So do your reps to exhaustion each time and increase the weight a little each week over a long time, and eat enough, a muscular diet, not too much but enough to see gains.
For the rest of your body never lift to exhaustion / failure this will result in bodybuilding hypertrophy which dosnt add power and greatly taxes cardio.