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.
nLastResend = GetTime();
    int relayed_tx_count = 0;
    { 
        LOCK(cs_wallet);
        
        for (std::pair<const uint256, CWalletTx>& item : mapWallet) {
            CWalletTx& wtx = item.second;
            
            
            if (wtx.nTimeReceived > m_best_block_time - 5 * 60) continue;
            relayed_tx_count += wtx.RelayWalletTransaction(locked_chain) ? 1 : 0;
        }
    } 
    if (relayed_tx_count > 0) {
        WalletLogPrintf("%s: rebroadcast %u unconfirmed transactions\n", __func__, relayed_tx_count);
    }
}
 
void MaybeResendWalletTxs()
{
    for (const std::shared_ptr<CWallet>& pwallet : GetWallets()) {
        auto locked_chain = pwallet->chain().lock();
        pwallet->ResendWalletTransactions(*locked_chain);
    }
}
/** @defgroup Actions
 *
 * @{
 */
CWallet::Balance CWallet::GetBalance(const int min_depth) const