I learned objective-c, then swift, then dart. I found the transition from swift to dart very easy. I was programming productively in a day or two

Technically you have everything you need to make complete apps for both ios and android. I have 3 complete apps in the stores with no objective-c/swift or java/kotlin code added.

The only reason if wouldn’t be a bad thing to learn the basics of swift is because you potentially could run into a client that requires device capabilities that can be accesses through talking to the native side. I would bookmark hackingwithswift.com, they have a guide that covers a ton of concepts with code in their beginner pdf. I believe it’s free on the the site with other more technical book available for a reasonable charge.