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 (block_hash == stop_block) {
                break;
            }
            {
                auto locked_chain = chain().lock();
                Optional<int> tip_height = locked_chain->getHeight();
                if (!tip_height || *tip_height <= block_height || !locked_chain->getBlockHeight(block_hash)) {
                    
                    
                    break;
                }
                
                block_hash = locked_chain->getBlockHash(++*block_height);
                progress_current = chain().guessVerificationProgress(block_hash);
                
                const uint256 prev_tip_hash = tip_hash;
                tip_hash = locked_chain->getBlockHash(*tip_height);
                if (stop_block.IsNull() && prev_tip_hash != tip_hash) {
                    
                    progress_end = chain().guessVerificationProgress(tip_hash);
                }
            }
        }
        ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()), 100); 
        if (block_height && fAbortRescan) {
            WalletLogPrintf("Rescan aborted at block %d. Progress=%f\n", *block_height, progress_current);
            result.status = ScanResult::USER_ABORT;
        } else if (block_height && chain().shutdownRequested()) {
            WalletLogPrintf("Rescan interrupted by shutdown request at block %d. Progress=%f\n", *block_height, progress_current);