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.
void UpdateTimeFirstKey(int64_t nCreateTime) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool AddCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret) override;
    
    bool LoadCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret);
    bool AddCScript(const CScript& redeemScript) override;
    bool LoadCScript(const CScript& redeemScript);
    
    bool AddDestData(const CTxDestination& dest, const std::string& key, const std::string& value) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool EraseDestData(const CTxDestination& dest, const std::string& key) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    void LoadDestData(const CTxDestination& dest, const std::string& key, const std::string& value) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool GetDestData(const CTxDestination& dest, const std::string& key, std::string* value) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    std::vector<std::string> GetDestValues(const std::string& prefix) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool AddWatchOnly(const CScript& dest, int64_t nCreateTime) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    bool RemoveWatchOnly(const CScript &dest) override EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool LoadWatchOnly(const CScript &dest);
    
    int64_t nRelockTime = 0;
    bool Unlock(const SecureString& strWalletPassphrase, bool accept_no_keys = false);
    bool ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase, const SecureString& strNewWalletPassphrase);