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.
{
    for (const CTxIn& txin : tx->vin) {
        auto it = mapWallet.find(txin.prevout.hash);
        if (it != mapWallet.end()) {
            it->second.MarkDirty();
        }
    }
}
bool CWallet::AbandonTransaction(interfaces::Chain::Lock& locked_chain, const uint256& hashTx)
{
    auto locked_chain_recursive = chain().lock();  
    LOCK(cs_wallet);
    WalletBatch batch(*database, "r+");
    std::set<uint256> todo;
    std::set<uint256> done;
    
    auto it = mapWallet.find(hashTx);
    assert(it != mapWallet.end());
    CWalletTx& origtx = it->second;
    if (origtx.GetDepthInMainChain(locked_chain) != 0 || origtx.InMempool()) {
        return false;
    }
    todo.insert(hashTx);
    while (!todo.empty()) {
        uint256 now = *todo.begin();
        todo.erase(now);
        done.insert(now);
        auto it = mapWallet.find(now);