What would be a rational subkey depth to use so that my address is infeasible to locate via a systematic search?
It's a bit strange to use single digits for the entire HD path since the BIP32 spec allows you to pick numbers up to 2^31 (non-hardened). BIP32 key derivation is largely using SHA512. Current ASICS for SHA256 can calculate about 10 trillion SHA256's per second, so about 160 quintillion hash operations in about 6 months. Assuming an ASIC can be built with roughly the same hashing power, you would need to go about 20 levels deep to force a machine like that to find your address in about 6 months. Of course, more machines mean faster cracking.

A far simpler solution would be to use the entire space of each level instead of just 10. 3 levels then would be plenty (~10^28)