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.
static const unsigned int MAX_GETDATA_SZ = 1000;
struct COrphanTx {
    
    CTransactionRef tx;
    NodeId fromPeer;
    int64_t nTimeExpire;
    size_t list_pos;
};
CCriticalSection g_cs_orphans;
std::map<uint256, COrphanTx> mapOrphanTransactions GUARDED_BY(g_cs_orphans);
void EraseOrphansFor(NodeId peer);
void Misbehaving(NodeId nodeid, int howmuch, const std::string& message="") EXCLUSIVE_LOCKS_REQUIRED(cs_main);
static constexpr unsigned int AVG_LOCAL_ADDRESS_BROADCAST_INTERVAL = 24 * 60 * 60;
static const unsigned int AVG_ADDRESS_BROADCAST_INTERVAL = 30;
/** Average delay between trickled inventory transmissions in seconds.
 *  Blocks and whitelisted receivers bypass this, outbound peers get half this delay. */
static const unsigned int INVENTORY_BROADCAST_INTERVAL = 5;
/** Maximum number of inventory items to send per transmission.
 *  Limits the impact of low-fee transaction floods. */
static constexpr unsigned int INVENTORY_BROADCAST_MAX = 7 * INVENTORY_BROADCAST_INTERVAL;
static constexpr unsigned int AVG_FEEFILTER_BROADCAST_INTERVAL = 10 * 60;
static constexpr unsigned int MAX_FEEFILTER_CHANGE_DELAY = 5 * 60;
namespace {
    
    int nSyncStarted GUARDED_BY(cs_main) = 0;
    /**
     * Sources of received blocks, saved to be able to send them reject
     * messages or ban them when processing happens afterwards.
     * Set mapBlockSource[hash].second to false if the node should not be
     * punished if the block is invalid.