The web's answers for a suitable antonym seem inappropriate [0] (all referring to "small" rather than "many parts". I reckon megalithic is the way to go.

Although I've been hosting my own mail server (postfix/dovecot) for years I must admit that I treat most of its parts as a single black box. My head just doesn't grok the entirety of processes and the various sub components (spam, grey listing, filters).

I prefer monolithic every day. It's not hard to provide plugins that allow custom filtering/ spam etc, so the end result can easily be the same.

Especially for personal /small business servers, simplicity wins everytime IMHO.

[0] https://www.merriam-webster.com/thesaurus/monolithic