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You need to format it as a array, with quotes around the whole thing so that K3s doesn't treat it as yaml

etcd-arg: "cipher-suites=[TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,TLS_RSA_WITH_AES_128_CBC_SHA256,TLS_RSA_WITH_AES_128_GCM_SHA256]"

seems like it worked, tnx