To provide a little more context here:
We do not currently have any plans to turn on mandatory async finalization in prod in the future. I wouldn't say that "async-only is looming". However, we may: turn it on for orders with many names; or turn it on for orders that require CAA rechecking due to relying on old validations; or turn it on for any order that takes more than 500ms to finalize; or turn it on for all orders during an emergency that is causing finalization to take unexpectedly long.
Because of this, we are keeping async finalization on in Staging for much the same reasons that we put a random key-value pair in the Directory: to encourage and require client agility. Asynchronous finalization has always been part of the RFC 8555 ACME specification; clients that do not implement it have always been time bombs lying in wait to cause issues for their operators. We ran smack into those problems when we first attempted to turn on async finalization; we are not willing to let clients ossify further.
Again, I'm sorry that this makes testing against staging difficult for your particular case. We are a small organization, and we cannot dedicate time and energy to supporting broken clients that have not had active development in over three years.