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.
{
    CNodeState::TxDownloadState& peer_download_state = state->m_tx_download;
    if (peer_download_state.m_tx_announced.size() >= MAX_PEER_TX_ANNOUNCEMENTS || peer_download_state.m_tx_announced.count(txid)) {
        
        
        return;
    }
    peer_download_state.m_tx_announced.insert(txid);
    int64_t process_time;
    int64_t last_request_time = GetTxRequestTime(txid);
    
    if (last_request_time == 0) {
        process_time = nNow;
    } else {
        
        
        process_time = last_request_time + GETDATA_TX_INTERVAL + GetRand(MAX_GETDATA_RANDOM_DELAY);
    }
    
    if (!state->fPreferredDownload) process_time += INBOUND_PEER_TX_DELAY;
    peer_download_state.m_tx_process_time.emplace(process_time, txid);
}
} 
void UpdateLastBlockAnnounceTime(NodeId node, int64_t time_in_seconds)