You would know your market way better than me, but just anecdotally I'm usually willing to pay $5 for a useful app. Based on your numbers it sounds like that would still make you money after 10 or so years. If the app is source available, I'll go up much higher. I'm definitely not a typical user, but for a "power" tool I'd think I'm in your market.
I know you exist. I'm actually able to find literally dozens of people like you, every month.
OTOH, for everyone of you (users willing to pay $1 for a tool that'll solve a problem they have every once in a while), I'm able to find 1000 that'll just install the ad supported app, use it that one time and forget about it (until the next time they need it).
Trust me: the barrier to get even $1 from a user for an app the majority uses once a week tops for a couple of minutes, is immense, especially if they can run it for free, in exchange for the annoyance of a single banner ad, that you can dismiss (and go full screen) after 30s, if you do use it longer than that.
But the free version must exist, for discoverablity if nothing else, and experience shows that degrading it to drive sales only jeopardizes total revenue. So ads it is.
