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.
{
        auto result = MakeUnique<LockingStateImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
        if (try_lock && result && !*result) return {};
        
        
        return std::move(result);
    }
    std::unique_ptr<Chain::Lock> assumeLocked() override { return MakeUnique<LockImpl>(); }
    bool findBlock(const uint256& hash, CBlock* block, int64_t* time, int64_t* time_max) override
    {
        CBlockIndex* index;
        {
            LOCK(cs_main);
            index = LookupBlockIndex(hash);
            if (!index) {
                return false;
            }
            if (time) {
                *time = index->GetBlockTime();
            }
            if (time_max) {
                *time_max = index->GetBlockTimeMax();
            }
        }
        if (block && !ReadBlockFromDisk(*block, index, Params().GetConsensus())) {
            block->SetNull();
        }
        return true;
    }
    void findCoins(std::map<COutPoint, Coin>& coins) override { return FindCoins(coins); }