Can democratic civilization progress past the point where societies become too complicated for ordinary people to understand?
No and yes.
If the people continue to reason, then no. An individual’s capacity to reason is acquired in problem solving, and civilization progresses though problem solving.
Unfortunately, civilization tends to want to infantilize its citizens. Power is acquired this way. It does this by addicting the people to cradle-to-grave entitlements. If this happens, people stop problem solving and their capacity to reason fails to develop.
Eventually the number of problem solvers in society is insufficient to sustain or continue the progress of civilization. In this case, the answer to your question is yes, and society begins to fall into decline.
This cycle has been observed before.
Thus they [societies] are always descending from good to bad and rising from bad to good. For virtue gives birth to quiet, quiet to leisure, leisure to disorder, disorder to ruin; and similarly, from ruin, order is born, from order, virtue; and from virtue, glory and good fortune.
I see it simply as the rise-fall-rise again of reason.