Theses are all theoretically helpful, seems varying degrees.
The most recent and promising is SR17018. An experimental drug, it seems to be able to help reduce tolerance and I believe offset withdrawals. Double check me on that, but there are some reports on reddit of this being very helpful for people addicted to potent drugs like fentanyl, the zenes, spirochlorphine, ultra potent opioids known to jack up tolerance quickly.
Second is ultra low dose naltrexone, or ULDN. I can't remember how it is supposed to work, but I believe it is helpful in reducing tolerance buildup. Not 100%, just slowing it down. I don't think it inherently reduce tolerance like SR17018, just reduce the build up rate. It was investigated to for distribution in a pill with oxycodone, but for some "odd" reason they didn't finish the trials IIRC. It's super low doses, I forget exactly but like 1 to 20 micrograms, not milligrams, micrograms a day.
NMDA antagonists. They supposedly help reduce the rate of tolerance build up as well. Seems memantine is a good choice, but if you don't have access, then dextromethorphan, the cough syrup ingredient, does the same thing. Consider taking a longer acting syrup like delsym, as that will keep the drug in your system longer.
Together, this can reduce the rate of tolerance build up and with the SR 17018, make it easy to reduce usage and quit.
I've wanted to see what happens if someone used all three from the start, and see how fast tolerance builds up.