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.
{
    AssertLockHeld(cs_wallet);
    UpdateTimeFirstKey(meta.nCreateTime);
    mapKeyMetadata[keyID] = meta;
}
void CWallet::LoadScriptMetadata(const CScriptID& script_id, const CKeyMetadata& meta)
{
    AssertLockHeld(cs_wallet);
    UpdateTimeFirstKey(meta.nCreateTime);
    m_script_metadata[script_id] = meta;
}
bool CWallet::WriteKeyMetadata(const CKeyMetadata& meta, const CPubKey& pubkey, const bool overwrite)
{
    return WalletBatch(*database).WriteKeyMetadata(meta, pubkey, overwrite);
}
void CWallet::UpgradeKeyMetadata()
{
    AssertLockHeld(cs_wallet);
    if (IsLocked() || IsWalletFlagSet(WALLET_FLAG_KEY_ORIGIN_METADATA)) {
        return;
    }
    std::unique_ptr<WalletBatch> batch = MakeUnique<WalletBatch>(*database);
    size_t cnt = 0;
    for (auto& meta_pair : mapKeyMetadata) {
        CKeyMetadata& meta = meta_pair.second;