For OSS graph visualization libraries, check out: - Sigma.js - Vis.js
For commercial options: - Ogma (disclosure: my company builds that one) - KeyLines
I've used it to map wireless trading networks with lots of connections.
The fact that you can make the nodes clickable to other webpages etc is a great feature for building "drill down" views.
Maybe helpful, more for visual graph data analysis than say arch diagramming: we build Graphistry specifically for interactively analyzing more data than these tools, eg, GPU client + GPU server, and being more batteries-included. The free Graphistry Hub site is similar to diagrams.net and GitHub for folks who aren't self-hosting GPUs.
We are still working towards OSS'ing the core renderer, but all our new graph neural networks + graph compute + graph layout layers are all already OSS-first (see PyGraphistry), and we are part of the apache arrow + rapids communities. If your kind of thing, we are very much hiring :)
Having used Graphviz extensively over the years as well, the cost of yFiles is certainly unfortunate, but I guess we simply serve a different markt segment than Graphviz or D3. (On a somewhat related note: Our online graph editor, yEd Live, support parsing a subset of Graphviz' syntax and that enables running at least some yFiles layout algorithms on Graphviz graphs. No export back, though.)