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.
if (gArgs.IsArgSet("-mintxfee")) {
        CAmount n = 0;
        if (!ParseMoney(gArgs.GetArg("-mintxfee", ""), n) || 0 == n) {
            chain.initError(AmountErrMsg("mintxfee", gArgs.GetArg("-mintxfee", "")));
            return nullptr;
        }
        if (n > HIGH_TX_FEE_PER_KB) {
            chain.initWarning(AmountHighWarn("-mintxfee") + " " +
                              _("This is the minimum transaction fee you pay on every transaction."));
        }
        walletInstance->m_min_fee = CFeeRate(n);
    }
    walletInstance->m_allow_fallback_fee = Params().IsFallbackFeeEnabled();
    if (gArgs.IsArgSet("-fallbackfee")) {
        CAmount nFeePerK = 0;
        if (!ParseMoney(gArgs.GetArg("-fallbackfee", ""), nFeePerK)) {
            chain.initError(strprintf(_("Invalid amount for -fallbackfee=<amount>: '%s'"), gArgs.GetArg("-fallbackfee", "")));
            return nullptr;
        }
        if (nFeePerK > HIGH_TX_FEE_PER_KB) {
            chain.initWarning(AmountHighWarn("-fallbackfee") + " " +
                              _("This is the transaction fee you may pay when fee estimates are not available."));
        }
        walletInstance->m_fallback_fee = CFeeRate(nFeePerK);
        walletInstance->m_allow_fallback_fee = nFeePerK != 0; 
    }
    if (gArgs.IsArgSet("-discardfee")) {
        CAmount nFeePerK = 0;
        if (!ParseMoney(gArgs.GetArg("-discardfee", ""), nFeePerK)) {
            chain.initError(strprintf(_("Invalid amount for -discardfee=<amount>: '%s'"), gArgs.GetArg("-discardfee", "")));