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.
const CWalletTx* GetWalletTx(const uint256& hash) const;
    
    bool CanSupportFeature(enum WalletFeature wf) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet) { AssertLockHeld(cs_wallet); return nWalletMaxVersion >= wf; }
    /**
     * populate vCoins with vector of available COutputs.
     */
    void AvailableCoins(interfaces::Chain::Lock& locked_chain, std::vector<COutput>& vCoins, bool fOnlySafe=true, const CCoinControl *coinControl = nullptr, const CAmount& nMinimumAmount = 1, const CAmount& nMaximumAmount = MAX_MONEY, const CAmount& nMinimumSumAmount = MAX_MONEY, const uint64_t nMaximumCount = 0, const int nMinDepth = 0, const int nMaxDepth = 9999999) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Return list of available coins and locked coins grouped by non-change output address.
     */
    std::map<CTxDestination, std::vector<COutput>> ListCoins(interfaces::Chain::Lock& locked_chain) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Find non-change parent output.
     */
    const CTxOut& FindNonChangeParentOutput(const CTransaction& tx, int output) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Shuffle and select coins until nTargetValue is reached while avoiding
     * small change; This method is stochastic for some inputs and upon
     * completion the coin set and corresponding actual target value is
     * assembled
     */
    bool SelectCoinsMinConf(const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, std::vector<OutputGroup> groups,