My reach is lower on spontaneous posting only. On scheduled, the longer I schedule for the better reach seems. The best is when there is a mix of both, scheduled and spontaneous. I think it’s because facebook and instagram both want to promote pages that are there for a long time so they will spend on ads too
A blend of spontaneous and scheduled posts will be a better option. By scheduling your posts you can use A/B testing to analyze in which specific time frame you are better achieving your goals. Moreover, sometimes a tweak of spontaneous posts will work good for you.
I personally think it might be how we create those posts when we are scheduling or posting spontaneously that affects the reach.
For example, scheduling allows us to be more thoughtful and careful when creating the post, the image, and the caption. So we might see better reach and engagement. But at the same time, if we schedule 10-20 posts at once, the quality might unintentionally fall because we might be creating posts for the sake of creating.
(Disclaimer: I work at Buffer. Generally, I’d encourage people to do both when appropriate. For example, schedule regular content and post spontaneously when there’s a good opportunity or at events.)