Go deep first
Get one agent doing one hard thing well, with real understanding and real checks, before you run several at once. A setup you can't trust with one agent gives you ten times the mess with ten.
Parallel agents multiply whatever you already have, and your ability to review doesn't multiply with them. The first deep run also teaches you what the checks and the brief need to say, which is what makes the later ones safe to leave alone.
Say it to your agent
“Don't fan this out yet. First do one migration end to end with me watching: find the real call sites, run the real checks, write down what went wrong. We parallelise the rest only once that one has gone through clean.”