Hello Sam and/or Max,
I'm French too and I read your blog sometimes ;-)
Is the following what you suggest:
You create an index, and the index creation takes 30 seconds. Then instead of writing directly to SQLite (which won't work since the index creation blocks other writers) you suggest to store the write in a queue (for example another SQLite database for durability), and have a worker apply the write to the main database when the index creation is done?