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 (::fPruneMode) {
            CBlockIndex* block = stop_height ? ::chainActive[*stop_height] : ::chainActive.Tip();
            while (block && block->nHeight >= start_height) {
                if ((block->nStatus & BLOCK_HAVE_DATA) == 0) {
                    return block->nHeight;
                }
                block = block->pprev;
            }
        }
        return nullopt;
    }
    Optional<int> findFork(const uint256& hash, Optional<int>* height) override
    {
        const CBlockIndex* block = LookupBlockIndex(hash);
        const CBlockIndex* fork = block ? ::chainActive.FindFork(block) : nullptr;
        if (height) {
            if (block) {
                *height = block->nHeight;
            } else {
                height->reset();
            }
        }
        if (fork) {
            return fork->nHeight;
        }
        return nullopt;
    }
    bool isPotentialTip(const uint256& hash) override