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 (mapKeyFirstBlock.empty())
        return;
    
    for (const auto& entry : mapWallet) {
        
        const CWalletTx &wtx = entry.second;
        if (Optional<int> height = locked_chain.getBlockHeight(wtx.hashBlock)) {
            
            for (const CTxOut &txout : wtx.tx->vout) {
                
                for (const auto &keyid : GetAffectedKeys(txout.scriptPubKey, *this)) {
                    
                    std::map<CKeyID, int>::iterator rit = mapKeyFirstBlock.find(keyid);
                    if (rit != mapKeyFirstBlock.end() && *height < rit->second)
                        rit->second = *height;
                }
            }
        }
    }
    
    for (const auto& entry : mapKeyFirstBlock)
        mapKeyBirth[entry.first] = locked_chain.getBlockTime(entry.second) - TIMESTAMP_WINDOW; 
}
/**
 * Compute smart timestamp for a transaction being added to the wallet.
 *
 * Logic:
 * - If sending a transaction, assign its timestamp to the current time.