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.
{
        LOCK(::mempool.cs);
        auto it = ::mempool.GetIter(txid);
        return it && (*it)->GetCountWithDescendants() > 1;
    }
    void relayTransaction(const uint256& txid) override
    {
        CInv inv(MSG_TX, txid);
        g_connman->ForEachNode([&inv](CNode* node) { node->PushInventory(inv); });
    }
    void getTransactionAncestry(const uint256& txid, size_t& ancestors, size_t& descendants) override
    {
        ::mempool.GetTransactionAncestry(txid, ancestors, descendants);
    }
    bool checkChainLimits(const CTransactionRef& tx) override
    {
        LockPoints lp;
        CTxMemPoolEntry entry(tx, 0, 0, 0, false, 0, lp);
        CTxMemPool::setEntries ancestors;
        auto limit_ancestor_count = gArgs.GetArg("-limitancestorcount", DEFAULT_ANCESTOR_LIMIT);
        auto limit_ancestor_size = gArgs.GetArg("-limitancestorsize", DEFAULT_ANCESTOR_SIZE_LIMIT) * 1000;
        auto limit_descendant_count = gArgs.GetArg("-limitdescendantcount", DEFAULT_DESCENDANT_LIMIT);
        auto limit_descendant_size = gArgs.GetArg("-limitdescendantsize", DEFAULT_DESCENDANT_SIZE_LIMIT) * 1000;
        std::string unused_error_string;
        LOCK(::mempool.cs);
        return ::mempool.CalculateMemPoolAncestors(entry, ancestors, limit_ancestor_count, limit_ancestor_size,
            limit_descendant_count, limit_descendant_size, unused_error_string);
    }
    CFeeRate estimateSmartFee(int num_blocks, bool conservative, FeeCalculation* calc) override