My current understanding is that excessive leverage caused the Great Recession.
Are there existing metrics for measuring the levels of leverage currently used in the US financial sector (lending, investing, bonds, etc.)?
The first place to start is the Federal Reserve’s Flow of Funds report (Z.1) - link to Fed website.
It has aggregate balance sheets on an instrument and sector basis (including the financial sector, as well as its components). One can then look at growth rates, or scale relative to national income/GDP.
Although I have personal preferences on what data I would look at, someone else might be able to suggest studies on particular indicators.