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.
#include <chainparams.h>
#include <init.h>
#include <interfaces/chain.h>
#include <net.h>
#include <scheduler.h>
#include <outputtype.h>
#include <util/system.h>
#include <util/moneystr.h>
#include <validation.h>
#include <walletinitinterface.h>
#include <wallet/rpcwallet.h>
#include <wallet/wallet.h>
#include <wallet/walletutil.h>
class WalletInit : public WalletInitInterface {
public:
    
    bool HasWalletSupport() const override {return true;}
    
    void AddWalletOptions() const override;
    
    bool ParameterInteraction() const override;
    
    void Construct(InitInterfaces& interfaces) const override;
};
const WalletInitInterface& g_wallet_init_interface = WalletInit();
void WalletInit::AddWalletOptions() const