It is very easy to be thinking about Callanish, or Avebury, or Stonehenge, of Arbor Low here. But a lot of circles are much smaller, they consist of small stones. There may have been a lot of thinking involved in the siting, layout and orientation (although a basic knowledge of astronomy and geometry may have been sufficient for that), but some of the circles would not have required a "massive undertaking". Take, for two random off the top of my head examples, Cerrig Pryfaid and Cerrig Duon in North and South Wales. A small number of people could have built these in a pretty short time.

The problem here is that, to answer the "ceremonial purposes?" question and apply it to stone circles generally rather than one or some in particular, a one size fits all approach isn't going to do it. You would need to demonstrate a commonality of intention between the builders of the super-monuments, which as you say would have involved a massive amount of resource, time and planning (although perhaps over a very long time-frame*) and the builders of the smaller upland circles.

I would think the only way of showing such a commonality of intention would be if there is something demonstrably linking them, like orientation, construction or alignment. For example, it's easy enough to presume a link between the RSCs of Aberdeenshire, they generally feature commonality of construction and orientation. But less easy to show how (if) they link to, say, Avebury, or Gors Fawr, or the Rollrights.

*We assume that Avebury was built in a short timescale, in "one go". But what does that actually mean? One stone erected every year would still be consistent with the timeframe for the monument's construction. How do we know it wasn't built over 100 years? Answer - we don't. And if you put up one stone a year, it isn't such a "massive undertaking" is it? I'm not suggesting this as a likely possibility, but it needs to be borne in mind that there are lots of variables here that we have no way of knowing.