は or が problem is hard to explain, though we unconsciously use them differently .
And we surely feel some difference between 桜の花が咲いてます and 桜の花は咲いてます.
桜の桜の花が咲いてます means just "The sakura trees are blooming" or "No other trees but the sakura trees are blooming." The emphasis may be put on the subject.
What do we see there? Answer: 桜が咲いてます
What kind of trees are blooming? Answer: 桜が咲いてます
桜の花は咲いてます, on the other hand, means "As for the sakura trees, they are blooming now." It gives information about the sakura trees.
How are they now? Answer: 桜は咲いてます
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Draw a contrast? I'm not sure...
Probably, new information and old information theory (I don't know the right term) can explain between は and が. In the sentence using は the subject is old information and the other is new information. In the sentence using が , the subject is new information and the other is old information, or the whole sentence is new information.