You should be able to define the external cipher suites for entrypoint (doc):
# Dynamic configuration
tls:
options:
default:
cipherSuites:
- TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
But it seems there is no dynamic config for the internal requests (doc, reference).
Note that you can check the changelog on Traefik Github.
Maybe open a feature request on Traefik Github?