I've been taking creatine for a few years now, but well under 5g/day. I started when I discovered I had one of the 'unfortunate' MTHFR SNPs making me a sub-optimal methylator. We humans can make creatine BUT the process consumes methyl groups. Taking some means I need to make that much less.

That said, as you mention, it's unclear it can make it past the BBB — something I’m finding to be surprisingly complex. To quote this abstract:

While there is substantial variability among the physical barriers of the CNS, the BBB generally consists of endothelial cells (ECs), pericytes, astrocytes, neurons, glial cells, and their associated basement membranes, which comprise a complex multilayer, multicellular structure known as the neurovascular unit (NVU).

As well, there are specialized lipoproteins in the involved epithelial cells, one of which is a research target for getting drugs past the BBB and so into the CNS.