I would suggest you to file a "MyCase" and ask if the multi-application bootloader can help you. To do so, at top of this page go to "Support & Community -> Technical Support -> Create a MyCase":
Have you consumed all the UDB blocks ? It might be possible to optimize on something else in your design. Can you share your project ? It is difficult to change from slave to master because all the necessary APIs are generated during compile time.
I've consumed all the sync cells...
I've opened a support case 2707901334.
Looks like I'll go with the dual bootloader approach.