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.
pfrom->orphan_work_set.insert(elem->first);
                    }
                }
            }
            pfrom->nLastTXTime = GetTime();
            LogPrint(BCLog::MEMPOOL, "AcceptToMemoryPool: peer=%d: accepted %s (poolsz %u txn, %u kB)\n",
                pfrom->GetId(),
                tx.GetHash().ToString(),
                mempool.size(), mempool.DynamicMemoryUsage() / 1000);
            
            ProcessOrphanTx(connman, pfrom->orphan_work_set, lRemovedTxn);
        }
        else if (fMissingInputs)
        {
            bool fRejectedParents = false; 
            for (const CTxIn& txin : tx.vin) {
                if (recentRejects->contains(txin.prevout.hash)) {
                    fRejectedParents = true;
                    break;
                }
            }
            if (!fRejectedParents) {
                uint32_t nFetchFlags = GetFetchFlags(pfrom);
                int64_t nNow = GetTimeMicros();
                for (const CTxIn& txin : tx.vin) {
                    CInv _inv(MSG_TX | nFetchFlags, txin.prevout.hash);
                    pfrom->AddInventoryKnown(_inv);
                    if (!AlreadyHave(_inv)) RequestTx(State(pfrom->GetId()), _inv.hash, nNow);
                }
                AddOrphanTx(ptx, pfrom->GetId());