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.
walletInstance->SetHDSeed(masterPubKey);
            hd_upgrade = true;
        }
        
        if (walletInstance->CanSupportFeature(FEATURE_HD_SPLIT)) {
            walletInstance->WalletLogPrintf("Upgrading wallet to use HD chain split\n");
            walletInstance->SetMinVersion(FEATURE_PRE_SPLIT_KEYPOOL);
            split_upgrade = FEATURE_HD_SPLIT > prev_version;
        }
        
        if (split_upgrade) {
            walletInstance->MarkPreSplitKeys();
        }
        
        if (hd_upgrade) {
            if (!walletInstance->TopUpKeyPool()) {
                chain.initError(_("Unable to generate keys"));
                return nullptr;
            }
        }
    }
    if (fFirstRun)
    {
        
        walletInstance->SetMinVersion(FEATURE_LATEST);
        if ((wallet_creation_flags & WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
            
            walletInstance->SetWalletFlag(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
        } else if (wallet_creation_flags & WALLET_FLAG_BLANK_WALLET) {
            walletInstance->SetWalletFlag(WALLET_FLAG_BLANK_WALLET);