You can run migrations from another process. Migrations are just writes, and SQLite supports writes from multiple processes.

A single transaction that does a very large write will likely impact the reader -- the reader will be blocked while the write finishes.

I use SQLite in a web scraper on my laptop. The scraper runs as 16 processes hammering the database, doing about 5,000 write transactions/sec. Occasionally, simple SELECT queries experience high latency (what would be a 1ms query takes 100-200ms), because they're blocked while the WAL gets checkpointed into the main database.

If you have much lower volume of writes, the checkpoint is smaller and so completes much faster, and so the worst case latency is much better. Since crawling is a non-interactive process, I don't care about the worst case latency. If I was writing a website, I'd feel differently--but most websites won't do 5,000 writes/second.