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.
{
    {
        LOCK(cs_wallet);
        for (std::pair<const uint256, CWalletTx>& item : mapWallet)
            item.second.MarkDirty();
    }
}
bool CWallet::MarkReplaced(const uint256& originalHash, const uint256& newHash)
{
    LOCK(cs_wallet);
    auto mi = mapWallet.find(originalHash);
    
    assert(mi != mapWallet.end());
    CWalletTx& wtx = (*mi).second;
    
    assert(wtx.mapValue.count("replaced_by_txid") == 0);
    wtx.mapValue["replaced_by_txid"] = newHash.ToString();
    WalletBatch batch(*database, "r+");
    bool success = true;
    if (!batch.WriteTx(wtx)) {
        WalletLogPrintf("%s: Updating batch tx %s failed\n", __func__, wtx.GetHash().ToString());
        success = false;
    }
    NotifyTransactionChanged(this, originalHash, CT_UPDATED);
    return success;
}
bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFlushOnClose)