This sounds like a hiring problem more than a technical challenge.
It's not reasonable for you to have to push Backend Developers to become Full-Stack developers, but that may be your only option unless you can get your management to replace the Frontend developer who left.
Another option for management is to hire a beginning Frontend developer instead of an experienced developer. I see this as preferable to converting a Backend developer, since a beginner can learn your exact stack without other team commitments to steal their time.
This sounds like a hiring problem more than a technical challenge.
It's not reasonable for you to have to push Backend Developers to become Full-Stack developers, but that may be your only option unless you can get your management to replace the Frontend developer who left.
Another option for management is to hire a beginning Frontend developer instead of an experienced developer. I see this as preferable to converting a Backend developer, since a beginner can learn your exact stack without other team commitments to steal their time.
The amount of frontend related issues goes up and down. We're kinda unique on that point 😅 But I get your point. Thank you for your comment.