Where can I download a snapshot of the Lightning network topology?
As far as I know currently noone explicitly provides this dataset / snapshot. However romperts recksplorer works with providing this dataset from lnd to the browser and have javascript visualize it. so you could do: wget https://rompert.com/networkgraphv2 but that is a little bit dirty.

The "easiest" way to achieve what you are looking for by doing it properly is probably to extract gossipd from lightningd. As you know c Lightning has this darmonized architecture so there gossipd should be able to run standalone, peer with nodes and download sync the network graph.

I might be interested in helping to run gossipd as a standalone tool. We could also just implement the gossip protocol ourselves as that part of lightning is pretty standalone.

Hope these two suggestions help