"Is no more than twice a week (so always at least 3-4 days in between doses) reasonable to achieve the sort of casual use that Im looking for?" -- IMO, yes, if you can stick to this schedule. Brainstorm and then figure out what safeguards you are going to realistically use to prevent yourself from saying fuck it, just this one time I'll go off schedule or use a little more.
Also, maybe it is worthwhile to try to find out why you want to use opiates regularly.
And maybe it is worthwhile to also pursue another hobby or venue/outlet that also gives you joy. Because opiate addictions often creep up on ppl and eventually addicts lose interest in hanging out with straight ppl and stop any activities not related to drugs.
My Experience:
I used to sniff oxycontin about 3 times a week for about 2.5 years, and then, I let myself slip and wound up physically addicted...and 6 years later...after losing everything and living through hell
I kicked the physical dependence to iv heroin and suboxone by using a pcp analog and methoxetamine (mxe). But whenever I used continuously (for me that means every 6 hours) for 3 days or more, I would find myself physically addicted again. So, then I'd use mxe again for 2 days and kick the opiate habit again.
Now, I still have the mental habit and crave relief from depression and boredom, so I will use about 1-3 shots about 1 or 2 times a week. I basically don't want to be physically addicted again and also lack the money and means to keep up a daily habit, and lack the self-control to save bags for next day use.
I am going to rehab soon and have a trip abroad, an intensive class, and school/internship lined up in hope that I won't even want to use opiates at all, with any kind of frequency.