Frankly, I have been an opiate user for more than 3 decades. Seen more than my fair share of ODs but I will say this, there are FAR more dangerous substances to attempt with. My only concern would be the difficulty in obtaining the right level of purity.
In order to obtain for a reliable source and to know you have a good quality product, you would really need some experience within the world of opiate usage.
Use of opiates has the wonderful effect of helping you give less-zero fucks about the very problems that caused you to feel this way in the first place.
Continued use of said opiates will cause a tolerance to build, which will render your method largely redundant. (Take this from someone who took a litre of methadone and 80,000mg quietapine with the result of nothing more than a solid night's sleep).
Of course, restraint from developing that dreaded tolerance, will result, upon the planned OD, with the mandatory opiate naive vomitting (which is virtually impossible to overcome, even with antiemetics).
Oh yeah!! Then there's the purity issue. If you are buying anything other than a medicinal product from a licensed supplier, there's simply no way to know if the quality is of any good standard. Dealers always tell you they're shit is "the bomb". They're not exactly going to try selling you something they openly admit is a pile of shite. Even dealers get stuck with bad deals from time to time. Then you need to be aware that every hand that stash passess through, has a tendency (unless terrified to do otherwise) to dip the bag, replacing the "dipped" area with something similar looking.
Of course, a medical product should be much more reliable in purity and consistency. But even then, especially depending upon from where you buy it, there remains doubt. Such substances are highly attractive to anyone through who's hands they might pass, because of the lucrative nature of the business.
Apart from that, yeah! Fantastic choice!