> I am a backend developer using Spring Boot and Java.

If your goal is speed to market use what you know which is Spring Boot and Java.

If your aim is to learn something new then go with Django or sprinkle in some Kotlin incrementally (eg tests). I don’t think it’ll matter in the long run which you choose.

Conflating I want to learn something new with I want to deliver quickly will give you a suboptimal outcome for both.