The `Broadcast`/`ResendWalletTransactions` interface is strange:

- it's the node saying to the wallet "Try to submit your unconfirmed transactions to the mempool again"
- it's called in one place by the node, in `SendMessages()` here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/net_processing.cpp#L3515. `SendMessages()` is called on the message handling thread on a loop.
- the actual scheduling of when the wallet sends messages is handled in the `ResendWalletTransactions()` function itself, using the `nNextResend` and `nLastResend` wallet globals (which could as easily be static variables in `ResendWalletTransactions()`).

Therefore the only use of `Broadcast()` is to periodically prod the wallet. The wallet itself decides whether it's time to resend transactions. This could be done by scheduling regular calls to `ResendWalletTransactions()`, in the same way that we schedule regular wallet flushes here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/wallet/init.cpp#L215.

A couple of minor implementation details:

- the call to `Broadcast()` in `SendMessages()` is protected by an if statement `if (!fReindex && !fImporting && !IsInitialBlockDownload()) {`. Those tests could be moved into `ResendWalletTransactions()` (through the `Chain` interface).
- The `Broadcast` notification passes in the `nTimeBestReceived` time of the best block. That could be fetched by `ResendWalletTransactions()` through the `Chain` interface.
if (internal) {
            chainChildKey.Derive(childKey, hdChain.nInternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
            metadata.hdKeypath = "m/0'/1'/" + std::to_string(hdChain.nInternalChainCounter) + "'";
            metadata.key_origin.path.push_back(0 | BIP32_HARDENED_KEY_LIMIT);
            metadata.key_origin.path.push_back(1 | BIP32_HARDENED_KEY_LIMIT);
            metadata.key_origin.path.push_back(hdChain.nInternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
            hdChain.nInternalChainCounter++;
        }
        else {
            chainChildKey.Derive(childKey, hdChain.nExternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
            metadata.hdKeypath = "m/0'/0'/" + std::to_string(hdChain.nExternalChainCounter) + "'";
            metadata.key_origin.path.push_back(0 | BIP32_HARDENED_KEY_LIMIT);
            metadata.key_origin.path.push_back(0 | BIP32_HARDENED_KEY_LIMIT);
            metadata.key_origin.path.push_back(hdChain.nExternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
            hdChain.nExternalChainCounter++;
        }
    } while (HaveKey(childKey.key.GetPubKey().GetID()));
    secret = childKey.key;
    metadata.hd_seed_id = hdChain.seed_id;
    CKeyID master_id = masterKey.key.GetPubKey().GetID();
    std::copy(master_id.begin(), master_id.begin() + 4, metadata.key_origin.fingerprint);
    metadata.has_key_origin = true;
    
    if (!batch.WriteHDChain(hdChain))
        throw std::runtime_error(std::string(__func__) + ": Writing HD chain model failed");
}
bool CWallet::AddKeyPubKeyWithDB(WalletBatch& batch, const CKey& secret, const CPubKey& pubkey)