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.
{
    return strprintf("COutput(%s, %d, %d) [%s]", tx->GetHash().ToString(), i, nDepth, FormatMoney(tx->tx->vout[i].nValue));
}
std::vector<CKeyID> GetAffectedKeys(const CScript& spk, const SigningProvider& provider)
{
    std::vector<CScript> dummy;
    FlatSigningProvider out;
    InferDescriptor(spk, provider)->Expand(0, DUMMY_SIGNING_PROVIDER, dummy, out);
    std::vector<CKeyID> ret;
    for (const auto& entry : out.pubkeys) {
        ret.push_back(entry.first);
    }
    return ret;
}
const CWalletTx* CWallet::GetWalletTx(const uint256& hash) const
{
    LOCK(cs_wallet);
    std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(hash);
    if (it == mapWallet.end())
        return nullptr;
    return &(it->second);
}
CPubKey CWallet::GenerateNewKey(WalletBatch &batch, bool internal)