Maybe you could grandfather people in that have bought it in the past six or twelve months? I’d be one of those customers, but I’m more than happy to make a donation to see you keep this going and would love to see new features developed. It’s a pretty cool package.
Going forward, the price point you’re at offers a great value (especially the unlimited site licensing). Asking for an annual renewal is absolutely fair and in line with what I’ve seen other developers do.