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.
bool EncryptWallet(const SecureString& strWalletPassphrase);
    void GetKeyBirthTimes(interfaces::Chain::Lock& locked_chain, std::map<CTxDestination, int64_t> &mapKeyBirth) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    unsigned int ComputeTimeSmart(const CWalletTx& wtx) const;
    /**
     * Increment the next transaction order id
     * @return next transaction order id
     */
    int64_t IncOrderPosNext(WalletBatch *batch = nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    DBErrors ReorderTransactions();
    void MarkDirty();
    bool AddToWallet(const CWalletTx& wtxIn, bool fFlushOnClose=true);
    void LoadToWallet(const CWalletTx& wtxIn) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void TransactionAddedToMempool(const CTransactionRef& tx) override;
    void BlockConnected(const CBlock& block, const std::vector<CTransactionRef>& vtxConflicted) override;
    void BlockDisconnected(const CBlock& block) override;
    void UpdatedBlockTip() override;
    int64_t RescanFromTime(int64_t startTime, const WalletRescanReserver& reserver, bool update);
    struct ScanResult {
        enum { SUCCESS, FAILURE, USER_ABORT } status = SUCCESS;
        
        
        
        uint256 last_scanned_block;
        Optional<int> last_scanned_height;
        
        
        
        
        uint256 last_failed_block;
    };