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.
reservekey.KeepKey();
            
            
            AddToWallet(wtxNew);
            
            for (const CTxIn& txin : wtxNew.tx->vin)
            {
                CWalletTx &coin = mapWallet.at(txin.prevout.hash);
                coin.BindWallet(this);
                NotifyTransactionChanged(this, coin.GetHash(), CT_UPDATED);
            }
        }
        
        
        CWalletTx& wtx = mapWallet.at(wtxNew.GetHash());
        if (fBroadcastTransactions)
        {
            
            if (!wtx.AcceptToMemoryPool(*locked_chain, state)) {
                WalletLogPrintf("CommitTransaction(): Transaction cannot be broadcast immediately, %s\n", FormatStateMessage(state));
                
            } else {
                wtx.RelayWalletTransaction(*locked_chain);
            }
        }
    }
    return true;
}
DBErrors CWallet::LoadWallet(bool& fFirstRunRet)