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.
SetServiceFlagsIBDCache(!fInitialDownload);
    if (!fInitialDownload) {
        
        std::vector<uint256> vHashes;
        const CBlockIndex *pindexToAnnounce = pindexNew;
        while (pindexToAnnounce != pindexFork) {
            vHashes.push_back(pindexToAnnounce->GetBlockHash());
            pindexToAnnounce = pindexToAnnounce->pprev;
            if (vHashes.size() == MAX_BLOCKS_TO_ANNOUNCE) {
                
                
                break;
            }
        }
        
        connman->ForEachNode([nNewHeight, &vHashes](CNode* pnode) {
            if (nNewHeight > (pnode->nStartingHeight != -1 ? pnode->nStartingHeight - 2000 : 0)) {
                for (const uint256& hash : reverse_iterate(vHashes)) {
                    pnode->PushBlockHash(hash);
                }
            }
        });
        connman->WakeMessageHandler();
    }
}
/**
 * Handle invalid block rejection and consequent peer banning, maintain which
 * peers announce compact blocks.
 */
void PeerLogicValidation::BlockChecked(const CBlock& block, const CValidationState& state) {