I’ve never seen any substantial evidence of a distro with outdated packages really being any more reliable than a rolling release.
I think the fundamental issue here is that you conflate the concepts of reliablility and stability. Those are not the same. Stability in distros is a question of how much they restrict change during support cycles in order to not be a moving target for developers and system integrators. Fundamentally a rolling release can't be stable. It can absolutely be reliable to use, but you wouldn't use it as a basis for an embedded system you're trying to develop.