How much better will I feel once I've started dialysis?

May be gradual . It depends and may vary from patient to patient. Some patients feel better as soon as dialysis is started and may not notice any change for a while. Some of it is psychological and depends on overall health.
Depends. It depends on how bad you feel now, how effective your dialysis treatments are, and how much, if any, residual kidney function you have. Your nephrologist should be able to give you the best answer.