From personal experience as a ministerial servant, there's never a quick clean way of leaving the job except by moving away. They need to keep worker bees. If you want to give a letter they'll try to focus on how they can help you stay on.
Initially, moving wasn't feasible for me when I was serving, so I basically just slacked off. They kept me on for quite some time though. I'm sure because nobody else wanted to do the accounts. They overlooked the fact that I never auxillary pioneered or stepped up for campaigns etc.
Since the whole sham is about keeping in the drones and keeping up numbers, you'll have to be substantially less than the congregation average for field service. Up to you how close to inactive you think you can get before it upsets family.
Again by experience: even with a combo of ~ 6 hrs per month and missing about half of the meetings ( in particular ones where you have parts), that'll get noticed. People will talk like they always do. The BOE will have to "delete" you, if you do that for a period of time spanning at least 2 CO visits. You're not helping with the load.
Be prepared for about 3/4 of the self righteous dubs to treat you like you were marked when they make the announcement. \
After that just move from congregation to congregation for different meetings (sporadically) and the fading should be on its way so long as you don't have persistent family nagging you.
