I can't speak to the neurontin b/c never tried it, but if you're avg daily dose is around 180mg of oxy, 2-4mg's of Bupe per day MIGHT be enough. (And if you think it will be enough it very well may, use the power of placebo to your advantage....) Normally, when people start Bupe w/ that kind of habit, they'll be put on 8mg's / day, divided into two 4mg doses. But start small w/ the Bupe initially, and work up in small increments only if absolutely necessary until you're ok. And if the 2-4mg's isn't cutting it, take up to 6-8mg's ur first day b/c that's when you'll need it the most, after 3 days, if you have about 10mgs of Bupe left, you will be OK doing 2mg's a day---you won't be great but this will let you stretch out your Bupe and it will be enough to handle the worst of your W/D's. (And I mean your first day on Bupe, NOT your first day sober....read below)
Also I'm sure you know to take the Bupe as far into your initial w/d period as possible so that: 1) you won't go into precipitated w/d, and 2) so that you can start at a lower dose and thus maximize the amount of days you get out of your 20mg's of Bupe.
The xanax will be helpful, but 1mg doesn't give you a lot of options, I'd say either take it when your w/d'ing before you start the Bupe to make that period easier, or before you go to bed the first night you start the Bupe, those will be the most difficult times and the xanax will help you sleep and calm your muscles, maybe even split it into two days if 0.5mg is enough for you (Neurontin may be helpful for these symptoms I just really don't know)
The tramadol will help, take it when you've finished your bupe, or b/c you have a lot, you can even take it w/ the Bupe when you begin to run out and are chipping away at Bupe pills.
Lastly, if you can buy Immodium, DO IT! I didn't believe it would work, was in a similar situation, and at high enough doses it actually freakin works, go figure. Also if you have a CC, order some poppy pods or Kratom, and good luck