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.
else
            ++it;
    }
    
    
    if (coin_control.m_avoid_partial_spends && vCoins.size() > OUTPUT_GROUP_MAX_ENTRIES) {
        
        
        
        Shuffle(vCoins.begin(), vCoins.end(), FastRandomContext());
    }
    std::vector<OutputGroup> groups = GroupOutputs(vCoins, !coin_control.m_avoid_partial_spends);
    size_t max_ancestors = (size_t)std::max<int64_t>(1, gArgs.GetArg("-limitancestorcount", DEFAULT_ANCESTOR_LIMIT));
    size_t max_descendants = (size_t)std::max<int64_t>(1, gArgs.GetArg("-limitdescendantcount", DEFAULT_DESCENDANT_LIMIT));
    bool fRejectLongChains = gArgs.GetBoolArg("-walletrejectlongchains", DEFAULT_WALLET_REJECT_LONG_CHAINS);
    bool res = nTargetValue <= nValueFromPresetInputs ||
        SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(1, 6, 0), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used) ||
        SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(1, 1, 0), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used) ||
        (m_spend_zero_conf_change && SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(0, 1, 2), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used)) ||
        (m_spend_zero_conf_change && SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(0, 1, std::min((size_t)4, max_ancestors/3), std::min((size_t)4, max_descendants/3)), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used)) ||
        (m_spend_zero_conf_change && SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(0, 1, max_ancestors/2, max_descendants/2), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used)) ||
        (m_spend_zero_conf_change && SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(0, 1, max_ancestors-1, max_descendants-1), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used)) ||
        (m_spend_zero_conf_change && !fRejectLongChains && SelectCoinsMinConf(nTargetValue - nValueFromPresetInputs, CoinEligibilityFilter(0, 1, std::numeric_limits<uint64_t>::max()), groups, setCoinsRet, nValueRet, coin_selection_params, bnb_used));
    
    util::insert(setCoinsRet, setPresetCoins);
    
    nValueRet += nValueFromPresetInputs;
    return res;
}
bool CWallet::SignTransaction(CMutableTransaction& tx)