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.
most_recent_block_hash = hashBlock;
        most_recent_block = pblock;
        most_recent_compact_block = pcmpctblock;
        fWitnessesPresentInMostRecentCompactBlock = fWitnessEnabled;
    }
    connman->ForEachNode([this, &pcmpctblock, pindex, &msgMaker, fWitnessEnabled, &hashBlock](CNode* pnode) {
        AssertLockHeld(cs_main);
        
        if (pnode->nVersion < INVALID_CB_NO_BAN_VERSION || pnode->fDisconnect)
            return;
        ProcessBlockAvailability(pnode->GetId());
        CNodeState &state = *State(pnode->GetId());
        
        
        if (state.fPreferHeaderAndIDs && (!fWitnessEnabled || state.fWantsCmpctWitness) &&
                !PeerHasHeader(&state, pindex) && PeerHasHeader(&state, pindex->pprev)) {
            LogPrint(BCLog::NET, "%s sending header-and-ids %s to peer=%d\n", "PeerLogicValidation::NewPoWValidBlock",
                    hashBlock.ToString(), pnode->GetId());
            connman->PushMessage(pnode, msgMaker.Make(NetMsgType::CMPCTBLOCK, *pcmpctblock));
            state.pindexBestHeaderSent = pindex;
        }
    });
}
/**
 * Update our best height and announce any block hashes which weren't previously
 * in chainActive to our peers.
 */
void PeerLogicValidation::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {
    const int nNewHeight = pindexNew->nHeight;
    connman->SetBestHeight(nNewHeight);