OP here. Some stuff that has been suggested to me in PM (various other communication channels):

- Mastodon documentation [1]

- Nextcloud whitepapers and case studies [2]

- Book on security of distributed systems: Security Engineering: A Guide to Building Dependable Distributed Systems [3]

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[1] https://docs.joinmastodon.org

[2] https://nextcloud.com/whitepapers/

[3] https://www.amazon.com/Security-Engineering-Building-Dependa...