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.
{
private:
    std::atomic<bool> fAbortRescan{false};
    std::atomic<bool> fScanningWallet{false}; 
    std::mutex mutexScanning;
    friend class WalletRescanReserver;
    WalletBatch *encrypted_batch GUARDED_BY(cs_wallet) = nullptr;
    
    int nWalletVersion GUARDED_BY(cs_wallet){FEATURE_BASE};
    
    int nWalletMaxVersion GUARDED_BY(cs_wallet) = FEATURE_BASE;
    int64_t nNextResend = 0;
    int64_t nLastResend = 0;
    bool fBroadcastTransactions = false;
    
    std::atomic<int64_t> m_best_block_time {0};
    /**
     * Used to keep track of spent outpoints, and
     * detect and report conflicts (double-spends or
     * mutated transactions where the mutant gets mined).
     */
    typedef std::multimap<COutPoint, uint256> TxSpends;
    TxSpends mapTxSpends GUARDED_BY(cs_wallet);
    void AddToSpends(const COutPoint& outpoint, const uint256& wtxid) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void AddToSpends(const uint256& wtxid) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Add a transaction to the wallet, or update it.  pIndex and posInBlock should
     * be set when the transaction was known to be included in a block.  When
     * pIndex == nullptr, then wallet state is not updated in AddToWallet, but