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 <utility>
#include <vector>
bool VerifyWallets(interfaces::Chain& chain, const std::vector<std::string>& wallet_files);
bool LoadWallets(interfaces::Chain& chain, const std::vector<std::string>& wallet_files);
void StartWallets(CScheduler& scheduler);
void FlushWallets();
void StopWallets();
void UnloadWallets();
void UnloadWallet(std::shared_ptr<CWallet>&& wallet);
bool AddWallet(const std::shared_ptr<CWallet>& wallet);
bool RemoveWallet(const std::shared_ptr<CWallet>& wallet);
bool HasWallets();
std::vector<std::shared_ptr<CWallet>> GetWallets();
std::shared_ptr<CWallet> GetWallet(const std::string& name);
std::shared_ptr<CWallet> LoadWallet(interfaces::Chain& chain, const WalletLocation& location, std::string& error, std::string& warning);
static const unsigned int DEFAULT_KEYPOOL_SIZE = 1000;
constexpr CAmount DEFAULT_PAY_TX_FEE = 0;
static const CAmount DEFAULT_FALLBACK_FEE = 20000;
static const CAmount DEFAULT_DISCARD_FEE = 10000;
static const CAmount DEFAULT_TRANSACTION_MINFEE = 1000;
static const CAmount WALLET_INCREMENTAL_RELAY_FEE = 5000;
static const bool DEFAULT_SPEND_ZEROCONF_CHANGE = true;
static const bool DEFAULT_WALLET_REJECT_LONG_CHAINS = false;
static const bool DEFAULT_AVOIDPARTIALSPENDS = false;
static const unsigned int DEFAULT_TX_CONFIRM_TARGET = 6;
static const bool DEFAULT_WALLET_RBF = false;
static const bool DEFAULT_WALLETBROADCAST = true;
static const bool DEFAULT_DISABLE_WALLET = false;
static constexpr size_t DUMMY_NESTED_P2WPKH_INPUT_SIZE = 91;
class CCoinControl;