Yes, you should book the outbound online as soon as they are available and call to add the return again, as soon as they are availiable.

The other option is book the return with full Avios if you have them, again when they are released, and then call to have 50% of the Avios refunded. You would have two bookings and 2 PNRs then.

This is a helpful read:

https://www.headforpoints.com/2025/01/03/how-do-british-airways-american-express-2-4-1-vouchers-work-3/