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.
}
    bool zapwallettxes = gArgs.GetBoolArg("-zapwallettxes", false);
    
    if (zapwallettxes && gArgs.SoftSetBoolArg("-persistmempool", false)) {
        LogPrintf("%s: parameter interaction: -zapwallettxes enabled -> setting -persistmempool=0\n", __func__);
    }
    
    if (zapwallettxes) {
        if (is_multiwallet) {
            return InitError(strprintf("%s is only allowed with a single wallet file", "-zapwallettxes"));
        }
        if (gArgs.SoftSetBoolArg("-rescan", true)) {
            LogPrintf("%s: parameter interaction: -zapwallettxes enabled -> setting -rescan=1\n", __func__);
        }
    }
    if (is_multiwallet) {
        if (gArgs.GetBoolArg("-upgradewallet", false)) {
            return InitError(strprintf("%s is only allowed with a single wallet file", "-upgradewallet"));
        }
    }
    if (gArgs.GetBoolArg("-sysperms", false))
        return InitError("-sysperms is not allowed in combination with enabled wallet functionality");
    if (gArgs.GetArg("-prune", 0) && gArgs.GetBoolArg("-rescan", false))
        return InitError(_("Rescans are not possible in pruned mode. You will need to use -reindex which will download the whole blockchain again."));
    if (::minRelayTxFee.GetFeePerK() > HIGH_TX_FEE_PER_KB)
        InitWarning(AmountHighWarn("-minrelaytxfee") + " " +
                    _("The wallet will avoid paying less than the minimum relay fee."));
    return true;
}
bool VerifyWallets(interfaces::Chain& chain, const std::vector<std::string>& wallet_files)