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.
bool ret;
                ret = reservekey.GetReservedKey(vchPubKey, true);
                if (!ret)
                {
                    strFailReason = _("Keypool ran out, please call keypoolrefill first");
                    return false;
                }
                const OutputType change_type = TransactionChangeType(coin_control.m_change_type ? *coin_control.m_change_type : m_default_change_type, vecSend);
                LearnRelatedScripts(vchPubKey, change_type);
                scriptChange = GetScriptForDestination(GetDestinationForKey(vchPubKey, change_type));
            }
            CTxOut change_prototype_txout(0, scriptChange);
            coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
            CFeeRate discard_rate = GetDiscardRate(*this);
            
            CFeeRate nFeeRateNeeded = GetMinimumFeeRate(*this, coin_control, &feeCalc);
            nFeeRet = 0;
            bool pick_new_inputs = true;
            CAmount nValueIn = 0;
            
            
            coin_selection_params.use_bnb = nSubtractFeeFromAmount == 0; 
            
            while (true)
            {
                nChangePosInOut = nChangePosRequest;
                txNew.vin.clear();
                txNew.vout.clear();
                bool fFirst = true;
                CAmount nValueToSelect = nValue;