I've never used it, but:

https://github.com/Vidi0/scratch-io

Says that it's a Rust-based command-line client, which is probably the sort of thing that I'd use myself, if I were on the hunt for a client, sorta analogous to lgogdownloader for GOG.

Note that it says:

Due to how the itch.io API works, it is not possible to update a game in-place. To update a game, you must remove it and install it again.