I do not know if your use of the Sphinx packet format is such that variable length routes are used... however you should know that your Sphinx packet implementation does leak the route length to the last hop.

According to the recently submitted paper: Breaking and (Partially) Fixing Provably Secure Onion Routing, ( https://arxiv.org/abs/1910.13772 ) there is a padding bug where a sphinx implementation creates a header and in so doing uses zeros to pad the beta field whereas random data should be used instead because of the use of the stream cipher...

https://github.com/UCL-InfoSec/sphinx/blob/c05b7034eaffd8f98454e0619b0b1548a9fa0f42/SphinxClient.py#L67

# The os.urandom used to be a string of 0x00 bytes, but that's wrong
beta = dest + id + os.urandom(((2 * (p.r - nu) + 2)*p.k - len(dest)))
beta = p.xor(beta,
p.rho(p.hrho(asbtuples[nu-1]['s']))[:(2*(p.r-nu)+3)*p.k]) + phi

This bug, that is, if beta were padded with zeros instead of random bytes would allow
an adversary to determine the length of the route. This adversary would have to be the last hop and it could then XOR the beta portion of the header with zero bytes to learn how many skipped hops the Sphinx packet had.