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.
{
    AssertLockHeld(cs_main);
    CNodeState &state = *State(pnode->GetId());
    if (enable_bip61) {
        for (const CBlockReject& reject : state.rejects) {
            connman->PushMessage(pnode, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, std::string(NetMsgType::BLOCK), reject.chRejectCode, reject.strRejectReason, reject.hashBlock));
        }
    }
    state.rejects.clear();
    if (state.fShouldBan) {
        state.fShouldBan = false;
        if (pnode->fWhitelisted)
            LogPrintf("Warning: not punishing whitelisted peer %s!\n", pnode->addr.ToString());
        else if (pnode->m_manual_connection)
            LogPrintf("Warning: not punishing manually-connected peer %s!\n", pnode->addr.ToString());
        else if (pnode->addr.IsLocal()) {
            
            LogPrintf("Warning: disconnecting but not banning local peer %s!\n", pnode->addr.ToString());
            pnode->fDisconnect = true;
        } else {
            
            if (m_banman) {
                m_banman->Ban(pnode->addr, BanReasonNodeMisbehaving);
            }
            connman->DisconnectNode(pnode->addr);
        }
        return true;
    }