When can I stop learning my target Asian language and still retain it?

I’m about B1 in J___ese (censored due to filter), and I’d like to learn my mother’s tongue which is also Asian and very similar in grammar and completely different vocab. However, I don’t want to mess up what I’ve learned in J already + I don’t believe I’ll have the time to study both adequately.

My original plan was to stop when I reach B2 and move on, but I’ve come to realize going from B1 to B2 in Asian languages (and probably others I’m sure, but especially Asian) takes waaaaay longer than A2 to B1. It might be 2 years before I’m B2 in an ideal timeline.

TLDR: I know B2 is generally accepted to be the “you won’t forget” level. But that’s gonna take way longer than I thought. Would it be advisable to drop the language now? Or is it too risky? Any advice is appreciated!