Having been through this a couple of times with commercial products, I think the very best answer is to use the native installer for each supported platform. Anything else produces an unpleasant experience for the end-user, and in practice you have to test on every platform you want to support anyway, so it’s not really a significant burden to maintain packages for each. The idea that you can create a binary that can “just work” on every platform out there, including some you’ve never even heard of, just really doesn’t work all that well.
My recommendation is that you pick a platform or two to support initially (Red Hat and Ubuntu would be my suggestions) and then let user demand drive the creation of additional installation packages. Perhaps make it known that you’re willing to support additional platforms, for a modest fee that covers your time and effort in packaging and testing on that platform. If a platform proves to be very different, you may need to charge more for ongoing support.
Oh, and I cannot overemphasize the value of virtual machines for scenarios like this. You need to build VMs for each platform you support, and perhaps multiple VMs per platform to make it easy to test different configurations.