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.
/** Get database handle used by this wallet. Ideally this function would
     * not be necessary.
     */
    WalletDatabase& GetDBHandle()
    {
        return *database;
    }
    /**
     * Select a set of coins such that nValueRet >= nTargetValue and at least
     * all coins from coinControl are selected; Never select unconfirmed coins
     * if they are not ours
     */
    bool SelectCoins(const std::vector<COutput>& vAvailableCoins, const CAmount& nTargetValue, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet,
                    const CCoinControl& coin_control, CoinSelectionParams& coin_selection_params, bool& bnb_used) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    const WalletLocation& GetLocation() const { return m_location; }
    /** Get a name for this wallet for logging/debugging purposes.
     */
    const std::string& GetName() const { return m_location.GetName(); }
    void LoadKeyPool(int64_t nIndex, const CKeyPool &keypool) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void MarkPreSplitKeys() EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    std::map<CKeyID, CKeyMetadata> mapKeyMetadata GUARDED_BY(cs_wallet);
    
    std::map<CScriptID, CKeyMetadata> m_script_metadata GUARDED_BY(cs_wallet);
    bool WriteKeyMetadata(const CKeyMetadata& meta, const CPubKey& pubkey, bool overwrite);
    typedef std::map<unsigned int, CMasterKey> MasterKeyMap;
    MasterKeyMap mapMasterKeys;
    unsigned int nMasterKeyMaxID = 0;
    
    CWallet(interfaces::Chain& chain, const WalletLocation& location, std::unique_ptr<WalletDatabase> database) : m_chain(chain), m_location(location), database(std::move(database))