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.
return (nValueRet >= nTargetValue);
    }
    
    std::set<CInputCoin> setPresetCoins;
    CAmount nValueFromPresetInputs = 0;
    std::vector<COutPoint> vPresetInputs;
    coin_control.ListSelected(vPresetInputs);
    for (const COutPoint& outpoint : vPresetInputs)
    {
        
        bnb_used = false;
        coin_selection_params.use_bnb = false;
        std::map<uint256, CWalletTx>::const_iterator it = mapWallet.find(outpoint.hash);
        if (it != mapWallet.end())
        {
            const CWalletTx& wtx = it->second;
            
            if (wtx.tx->vout.size() <= outpoint.n)
                return false;
            
            nValueFromPresetInputs += wtx.tx->vout[outpoint.n].nValue;
            setPresetCoins.insert(CInputCoin(wtx.tx, outpoint.n));
        } else
            return false; 
    }
    
    for (std::vector<COutput>::iterator it = vCoins.begin(); it != vCoins.end() && coin_control.HasSelected();)
    {
        if (setPresetCoins.count(it->GetInputCoin()))
            it = vCoins.erase(it);