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.
if (nSubtractFeeFromAmount == 0)
                    nValueToSelect += nFeeRet;
                
                coin_selection_params.tx_noinputs_size = 11; 
                for (const auto& recipient : vecSend)
                {
                    CTxOut txout(recipient.nAmount, recipient.scriptPubKey);
                    if (recipient.fSubtractFeeFromAmount)
                    {
                        assert(nSubtractFeeFromAmount != 0);
                        txout.nValue -= nFeeRet / nSubtractFeeFromAmount; 
                        if (fFirst) 
                        {
                            fFirst = false;
                            txout.nValue -= nFeeRet % nSubtractFeeFromAmount;
                        }
                    }
                    
                    coin_selection_params.tx_noinputs_size += ::GetSerializeSize(txout, PROTOCOL_VERSION);
                    if (IsDust(txout, chain().relayDustFee()))
                    {
                        if (recipient.fSubtractFeeFromAmount && nFeeRet > 0)
                        {
                            if (txout.nValue < 0)
                                strFailReason = _("The transaction amount is too small to pay the fee");
                            else
                                strFailReason = _("The transaction amount is too small to send after the fee has been deducted");
                        }
                        else
                            strFailReason = _("Transaction amount too small");