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.
/**
     * Notifies listeners of a block being connected.
     * Provides a vector of transactions evicted from the mempool as a result.
     *
     * Called on a background thread.
     */
    virtual void BlockConnected(const std::shared_ptr<const CBlock> &block, const CBlockIndex *pindex, const std::vector<CTransactionRef> &txnConflicted) {}
    /**
     * Notifies listeners of a block being disconnected
     *
     * Called on a background thread.
     */
    virtual void BlockDisconnected(const std::shared_ptr<const CBlock> &block) {}
    /**
     * Notifies listeners of the new active block chain on-disk.
     *
     * Prior to this callback, any updates are not guaranteed to persist on disk
     * (ie clients need to handle shutdown/restart safety by being able to
     * understand when some updates were lost due to unclean shutdown).
     *
     * When this callback is invoked, the validation changes done by any prior
     * callback are guaranteed to exist on disk and survive a restart, including
     * an unclean shutdown.
     *
     * Provides a locator describing the best chain, which is likely useful for
     * storing current state on disk in client DBs.
     *
     * Called on a background thread.
     */
    virtual void ChainStateFlushed(const CBlockLocator &locator) {}