If you're interested in how the modern Agency functions, I'd suggest reading The DARPA Model for Transformative Technologies.
It describes some of the typical trends in our time: contracts too often go to entrenched players and primes, rather than startups as primes due to technical merit rather than past performance and relationships. DARPA personnel reaching out from anonymous mailboxes post-RFI submission, asking if your team has TS/Sci clearance. (No answer if it's not a complete 100% yes.) And landlordism: extracting value from an existing system rather than seeding many new technologies.
However, there is still nothing like DARPA, despite its many problems. DIU is more for near-term innovation, and had its budget cut 20% just now. AFRL, AFWERX, Army SBIR/STTR, and so on, are all critical but not built to prevent technological surprise. Thank your lucky stars we have DARPA still up and running after so many decades.