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.
unsigned int nMessageSize = hdr.nMessageSize;
    
    CDataStream& vRecv = msg.vRecv;
    const uint256& hash = msg.GetMessageHash();
    if (memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) != 0)
    {
        LogPrint(BCLog::NET, "%s(%s, %u bytes): CHECKSUM ERROR expected %s was %s\n", __func__,
           SanitizeString(strCommand), nMessageSize,
           HexStr(hash.begin(), hash.begin()+CMessageHeader::CHECKSUM_SIZE),
           HexStr(hdr.pchChecksum, hdr.pchChecksum+CMessageHeader::CHECKSUM_SIZE));
        return fMoreWork;
    }
    
    bool fRet = false;
    try
    {
        fRet = ProcessMessage(pfrom, strCommand, vRecv, msg.nTime, chainparams, connman, interruptMsgProc, m_enable_bip61);
        if (interruptMsgProc)
            return false;
        if (!pfrom->vRecvGetData.empty())
            fMoreWork = true;
    }
    catch (const std::ios_base::failure& e)
    {
        if (m_enable_bip61) {
            connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_MALFORMED, std::string("error parsing message")));
        }
        if (strstr(e.what(), "end of data"))
        {