You can ease the ride with codeine, but substituting with other opiates is simply prolonging the agony and risking swapping one addiction for another. At your stage, you would be well advised to just tough it out. The first rattle I had which I mentioned above was from 0.1g/day for about a year - so not that dissimilar to your habit. It was still fuckin horrible, but I managed to carry on going to work every day. The worst bit was not sleeping and the dreaded night sweats and RLS.
I disagree personally. After coming off oxy - and this was for the fifth or so time with a tolerance so high that ~300mg did fuck all, so you can imagine this was no baby rattle - a bottle of codeine linctus helped me big time to ease myself off after jumping from my oxy taper. It is weak enough that it won't become a habit in itself if you have oxy/smack tolerance because you won't get high off it, but it has enough of an effect that it is good at taking the edge off the withdrawals.
This of course is assuming you use the codeine to taper, so you gotta actually reduce your dose daily, not just start doing codeine instead. But this is easy to do when your tolerance precludes you from getting high on codeine in the first place.
Once the codeine runs out, you should be fine with only very mild symptoms especially on the first rattle. If insomnia, RLS, sweats, anxiety, depression etc become an issue, lope helps the physical symptoms while pregabalin or benzos help the insomnia and mental stuff, but I will say there is a true risk of replacing one habit with another if you use benzos and pregabs so they should be a last resort.
Cannabis is a great helper in withdrawal that doesn't carry the risk of landing you in another bad habit. I recommend that for sure. A sedating indica is great for sleep and anxiety.
Oh and most likely your sex drive will suddenly come back with a vengeance which also helps. Right after withdrawal you will probably not last very long though so I recommend just wanking when the mood takes you. The chemicals released in your brain relax you naturally and can help you sleep.
But honestly I suspect most of OP's issues will be psychological. I never got bad withdrawals even off oxy during my first rattle or two. It's only when getting on and off it became a pattern that I started to feel the consequences.