In case you haven't already explored existing browser alternatives, [Qutebrowser](https://qutebrowser.org/) is a keyboard-driven browser that can be infinitely extended as it is written in Python and you have full access to the browser API (including the low-level internals) in your configuration file. You can import any Python modules you need, and it has native support for ad blocker lists + userscripts (Greasemonkey et al).