I cut back on caffeine a few months ago, but didn't eliminate it entirely for the reason you note in your post: I'm way sensitive to it. So I decided to keep a modest immunity going. My own iocane strategy, you might say.
I'm down to one cup in the morning (plus any chocolate I eat during the day), but I've toyed with adding in a half-cup in the afternoon (both for the renewed boost and for all-day immunity; apparently withdrawal symptoms become evident within 12 hours, though I can't say I've noticed them). That might be the perfect caffeine intake for me, I think.
I wonder if there's a way for you to set up a Beeminder goal that keeps you in your caffeine sweet spot? (If you've any interest in pursuing it further.)
And thanks for the unexpected and fortuitous influx of cash. It's been put to excellent use.