You could do the same as many OSS projects - offer an up-to-date version and an LTS version, both with clear schedules. If the LTS is stable - nothing except security or crucial bug fixes - hopefully you won't lose much time supporting it.
The fun is the LTS users will want 95% of the bug fixes and 30% of the features only in the current version. Oh and each customer wants a different 30% of the features. So you need to build them a custom branch or they quit.
