I have triple-checked on PolygonScan that the transaction was successful. However, in metamask and in Dapps, the balance shows as 0.
I’m experienced with Defi, so this is not a newbie problem.
I do not see how we can be of help here. You need to contact Coinbase and make sure that everything is ok on their side if the address sent is correct.
You can also make sure your wallet is on the correct network and change your RPC to a different provider. Not necessarily in this order 