As I understand your challenge
- dual boot Manjaro Linux and Windows
- Encrypted Manjaro Linux
- Secure Boot enabled
- A loader allowing to select Windows or Manjaro
I have done the following out of curiosity - could it be done - at all?
The goal was to identify what need to be changed to be able to achieve what you are trying to do.
I have created a system configuration similar to your setup using a laptop with two disks.
I installed a Windows 10 on first disk - then proceeded to create an encrypted Manjaro installation on the second disk using Calamares - selecting btrfs and swap to file.
While I do have some experience in doing this from scratch I have never tried to convert a Manjaro Linux encrypted setup - but I did - and it works ..
I made notes on the changes - I can share them if there is interest ..