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 (wtx.tx->vout[i].nValue < nMinimumAmount || wtx.tx->vout[i].nValue > nMaximumAmount)
                continue;
            if (coinControl && coinControl->HasSelected() && !coinControl->fAllowOtherInputs && !coinControl->IsSelected(COutPoint(entry.first, i)))
                continue;
            if (IsLockedCoin(entry.first, i))
                continue;
            if (IsSpent(locked_chain, wtxid, i))
                continue;
            isminetype mine = IsMine(wtx.tx->vout[i]);
            if (mine == ISMINE_NO) {
                continue;
            }
            bool solvable = IsSolvable(*this, wtx.tx->vout[i].scriptPubKey);
            bool spendable = ((mine & ISMINE_SPENDABLE) != ISMINE_NO) || (((mine & ISMINE_WATCH_ONLY) != ISMINE_NO) && (coinControl && coinControl->fAllowWatchOnly && solvable));
            vCoins.push_back(COutput(&wtx, i, nDepth, spendable, solvable, safeTx, (coinControl && coinControl->fAllowWatchOnly)));
            
            if (nMinimumSumAmount != MAX_MONEY) {
                nTotal += wtx.tx->vout[i].nValue;
                if (nTotal >= nMinimumSumAmount) {
                    return;
                }
            }
            
            if (nMaximumCount > 0 && vCoins.size() >= nMaximumCount) {
                return;
            }
        }
    }
}
std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins(interfaces::Chain::Lock& locked_chain) const