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.
} else if (!wallet_dir.is_absolute()) {
            chain.initError(strprintf(_("Specified -walletdir \"%s\" is a relative path"), wallet_dir.string()));
            return false;
        }
        gArgs.ForceSetArg("-walletdir", canonical_wallet_dir.string());
    }
    LogPrintf("Using wallet directory %s\n", GetWalletDir().string());
    chain.initMessage(_("Verifying wallet(s)..."));
    
    
    
    bool salvage_wallet = gArgs.GetBoolArg("-salvagewallet", false) && wallet_files.size() <= 1;
    
    std::set<fs::path> wallet_paths;
    for (const auto& wallet_file : wallet_files) {
        WalletLocation location(wallet_file);
        if (!wallet_paths.insert(location.GetPath()).second) {
            chain.initError(strprintf(_("Error loading wallet %s. Duplicate -wallet filename specified."), wallet_file));
            return false;
        }
        std::string error_string;
        std::string warning_string;
        bool verify_success = CWallet::Verify(chain, location, salvage_wallet, error_string, warning_string);
        if (!error_string.empty()) chain.initError(error_string);
        if (!warning_string.empty()) chain.initWarning(warning_string);
        if (!verify_success) return false;
    }
    return true;
}
void WalletInit::Construct(InitInterfaces& interfaces) const