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.
ScanResult ScanForWalletTransactions(const uint256& first_block, const uint256& last_block, const WalletRescanReserver& reserver, bool fUpdate);
    void TransactionRemovedFromMempool(const CTransactionRef &ptx) override;
    void ReacceptWalletTransactions(interfaces::Chain::Lock& locked_chain) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void ResendWalletTransactions(interfaces::Chain::Lock& locked_chain);
    struct Balance {
        CAmount m_mine_trusted{0};           
        CAmount m_mine_untrusted_pending{0}; 
        CAmount m_mine_immature{0};          
        CAmount m_watchonly_trusted{0};
        CAmount m_watchonly_untrusted_pending{0};
        CAmount m_watchonly_immature{0};
    };
    Balance GetBalance(int min_depth = 0) const;
    CAmount GetAvailableBalance(const CCoinControl* coinControl = nullptr) const;
    OutputType TransactionChangeType(OutputType change_type, const std::vector<CRecipient>& vecSend);
    /**
     * Insert additional inputs into the transaction by
     * calling CreateTransaction();
     */
    bool FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nChangePosInOut, std::string& strFailReason, bool lockUnspents, const std::set<int>& setSubtractFeeFromOutputs, CCoinControl);
    bool SignTransaction(CMutableTransaction& tx) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Create a new transaction paying the recipients with a set of coins
     * selected by SelectCoins(); Also create the change output, when needed
     * @note passing nChangePosInOut as -1 will result in setting a random position
     */
    bool CreateTransaction(interfaces::Chain::Lock& locked_chain, const std::vector<CRecipient>& vecSend, CTransactionRef& tx, CReserveKey& reservekey, CAmount& nFeeRet, int& nChangePosInOut,
                           std::string& strFailReason, const CCoinControl& coin_control, bool sign = true);
    bool CommitTransaction(CTransactionRef tx, mapValue_t mapValue, std::vector<std::pair<std::string, std::string>> orderForm, CReserveKey& reservekey, CValidationState& state);
    bool DummySignTx(CMutableTransaction &txNew, const std::set<CTxOut> &txouts, bool use_max_sig = false) const