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How is this different from Vaadin (other than the language, of course)?

That's a pretty big difference though.

Would also like to know major differences to Anvil?

Anvil is hosted, or an on-prem server, whereas it looks like this can be run anywhere. I’d guess that’s the major difference.

Anvil also emphasises their dev tools, it’s quite an integrated environment, between IDE, data, and deployments.