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.
{
        fCreditCached = false;
        fAvailableCreditCached = false;
        fImmatureCreditCached = false;
        fWatchDebitCached = false;
        fWatchCreditCached = false;
        fAvailableWatchCreditCached = false;
        fImmatureWatchCreditCached = false;
        fDebitCached = false;
        fChangeCached = false;
    }
    void BindWallet(CWallet *pwalletIn)
    {
        pwallet = pwalletIn;
        MarkDirty();
    }
    
    CAmount GetDebit(const isminefilter& filter) const;
    CAmount GetCredit(interfaces::Chain::Lock& locked_chain, const isminefilter& filter) const;
    CAmount GetImmatureCredit(interfaces::Chain::Lock& locked_chain, bool fUseCache=true) const;
    
    
    
    
    CAmount GetAvailableCredit(interfaces::Chain::Lock& locked_chain, bool fUseCache=true, const isminefilter& filter=ISMINE_SPENDABLE) const NO_THREAD_SAFETY_ANALYSIS;
    CAmount GetImmatureWatchOnlyCredit(interfaces::Chain::Lock& locked_chain, const bool fUseCache=true) const;
    CAmount GetChange() const;
    
    int GetSpendSize(unsigned int out, bool use_max_sig = false) const