This isn't a direct answer to the question, but being Canadian, I feel an intrinsic need to address the presumptions in this question which I view as overly US-centric.
Which slightly peculiar, as "constitutional" acts often precede the majority of a republic's other active statutes, and come reasonably early in the republic's foundation.
This isn't that peculiar. Countries with "late" and "major" constitutional documents include Canada (1982), Egypt (2014), France (arguably the 2008 amendments), New Zealand (1990), Norway (2014), Sweden (2011). Yes a lot of these aren't republics, but neither is the comparison country of the UK.
The UK is in the peculiar position of not in fact being a republic, yet still featuring many of the elements of modern democracies
Again, this is not too peculiar. Reasonably democratic non-republics include: Australia, Belgium, Canada, Japan, the Netherlands, New Zealand, Norway, Spain, Sweden.
certain acts are held up to be "constitutional" (ie supreme).
The "ie supreme" parenthetical can be misleading. Constitutional documents are not necessarily supreme in the sense of automatically overriding other laws or requiring some sort of super-majority to amend or repeal (both true in the US, neither true in the UK). Admittedly, of my issues with the presumptions in this question, this one is more often correct than the others I pointed out, but other examples I'm aware of are: the Netherlands, New Zealand, and to an extent Switzerland (in terms of not automatically overriding other laws).