Well, the consensus is the ultimate root of trust for the Tor network. Sample: http://171.25.193.9:443/tor/status-vote/current/consensus
It's a very large ASCII document, and you'd need to hardcode one or more DirAuth keys. But it has a timestamp in it. You could provide older consensuses to the smartcard and get it to sign historical data, but not future ones.
Before choosing the consensus, I would see which DirAuth-signed document would be best to use. Consensus and microconsensus are two options. I'm nto sure if there's anything else smaller the DirAuths sign... MicroDescriptors? (Unsure.)
-tom