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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
It took a lot of clicking, but they have a GitHub org, too: https://github.com/holistics/dbml#readme
Over the course of my career I've been pretty database centric in the work that I do. The databases I deal with will typically be highly normalized with table counts from around 300 on the low end and just over 1000 on the higher end. My experience is that they are helpful, though they can sometimes be a little overrated. I don't think I'd call them essential. Depending on the database vendor and the how the database was developed, if certain documentation was built into the database schema, some diagramming tools can pull this into the right places and so surface that documentation in way that can be more fluidly examined (think PostgreSQL's COMMENT ON functionality).
As others have pointed out, I usually don't use them to try to get the big picture: you're right in it is a morass of over-lapping lines and tiny boxes. At the whole database view, the best you can hope for is finding certain "information clusters", places where many references cluster, that can get you to the major ideas implemented by the application; this is helpful sometimes when coming to an application you haven't encountered before. The more useful aspect is when you've already got a central table and you're trying to understand the closest relationships to that table: maybe directly related or one away. There are database diagramming tools that will allow you to pick a relation and then limit the diagram to those one away, two away sets of relationships. Of course manually just telling the tool to diagram only a particular subject area of the database at a time reduces the sense of noise in these diagrams and typically you're only really trying to understand the data structure of such a subset of the database at any one time anyway.
The open source diagramming tool I use is called SchemaSpy (https://schemaspy.org/). Its not a database design GUI or anything like that. It inspects an existing database and creates documentation based on what it finds. It does that "pick a relation and show me relationships a couple degrees of separation" thing.