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.
send = false;
    }
    
    
    if (send && (pindex->nStatus & BLOCK_HAVE_DATA))
    {
        std::shared_ptr<const CBlock> pblock;
        if (a_recent_block && a_recent_block->GetHash() == pindex->GetBlockHash()) {
            pblock = a_recent_block;
        } else if (inv.type == MSG_WITNESS_BLOCK) {
            
            
            std::vector<uint8_t> block_data;
            if (!ReadRawBlockFromDisk(block_data, pindex, chainparams.MessageStart())) {
                assert(!"cannot load block from disk");
            }
            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::BLOCK, MakeSpan(block_data)));
            
        } else {
            
            std::shared_ptr<CBlock> pblockRead = std::make_shared<CBlock>();
            if (!ReadBlockFromDisk(*pblockRead, pindex, consensusParams))
                assert(!"cannot load block from disk");
            pblock = pblockRead;
        }
        if (pblock) {
            if (inv.type == MSG_BLOCK)
                connman->PushMessage(pfrom, msgMaker.Make(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::BLOCK, *pblock));
            else if (inv.type == MSG_WITNESS_BLOCK)
                connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::BLOCK, *pblock));