I am in the process of working on a decentralized application using IPFS and OrbitDB. IPFS uses 2048 bit RSA keys for the Node runtime peer-id and secp256k1 for read/write access in OrbitDB. For secp256k1 there exists a method for deterministically generating keys (bip39 mnemonic phrase).
Let's say I take an existing RSA implementation and replace the RNG with a seeded RNG which is seeded with the mnemonic phrase from above. Assuming I didn't roll my own implementation, aside from swapping out the RNG, how secure would this be? Are there any obvious flaws with this approach?