Azure Data Studio can connect to a variety of databases and has completions, but tend to forget if you've set a cell to output a plot. It also doesn't have good functionality for carrying over results from one cell to the next.
Jupyter notebooks don't have any kind of autocompletion against a database (at least to my knowledge), but you do get a lot of control of how you want to store things between cells and display things.
This DuckDB UI looks great, and while DuckDB can read a lot of files, I'm not sure if it has enough connectors to be a general database exploration notebook