> What about mixed delete/write/read workloads?

The source code to the test program is part of the SQLite source tree. It should be fairly easy to modify it to test the scheme you have in mind.

> How well does SQLite deal with fragmentation?

SQLite operates a lot like a modern filesystem: tree-based indexing structures, writes go to empty spaces, deletes leave holes that may later be filled by inserts, etc.

SQLite has active mechanisms to reduce fragmentation on the fly: b-tree page defragmentation (https://www.sqlite.org/fileformat.html) and auto-vaccuum (https://www.sqlite.org/pragma.html#pragma_auto_vacuum) being the main ones. If the DB file gets too fragmented, you can run a manual VACCUM (https://www.sqlite.org/lang_vacuum.html) on it.