I like May after Golden Week, but October / November is also nice (albeit potentially busier with the attraction of Halloween theming, merchandise, etc).
If you're interested in Fantasy Springs at DisneySea, then visiting as long as possible after its opening in June 2024, is going to potentially make your visit more pleasant (although I would expect crowd levels in Fantasy Springs to be high for many years to come based on how crowded Mario Land at USJ remains).
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