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.
{
    gArgs.AddArg("-addresstype", strprintf("What type of addresses to use (\"legacy\", \"p2sh-segwit\", or \"bech32\", default: \"%s\")", FormatOutputType(DEFAULT_ADDRESS_TYPE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-avoidpartialspends", strprintf("Group outputs by address, selecting all or none, instead of selecting on a per-output basis. Privacy is improved as an address is only used once (unless someone sends to it after spending from it), but may result in slightly higher fees as suboptimal coin selection may result due to the added limitation (default: %u)", DEFAULT_AVOIDPARTIALSPENDS), false, OptionsCategory::WALLET);
    gArgs.AddArg("-changetype", "What type of change to use (\"legacy\", \"p2sh-segwit\", or \"bech32\"). Default is same as -addresstype, except when -addresstype=p2sh-segwit a native segwit output is used when sending to a native segwit address)", false, OptionsCategory::WALLET);
    gArgs.AddArg("-disablewallet", "Do not load the wallet and disable wallet RPC calls", false, OptionsCategory::WALLET);
    gArgs.AddArg("-discardfee=<amt>", strprintf("The fee rate (in %s/kB) that indicates your tolerance for discarding change by adding it to the fee (default: %s). "
                                                                "Note: An output is discarded if it is dust at this rate, but we will always discard up to the dust relay fee and a discard fee above that is limited by the fee estimate for the longest target",
                                                              CURRENCY_UNIT, FormatMoney(DEFAULT_DISCARD_FEE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data (default: %s)",
                                                               CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u)", DEFAULT_KEYPOOL_SIZE), false, OptionsCategory::WALLET);
    gArgs.AddArg("-mintxfee=<amt>", strprintf("Fees (in %s/kB) smaller than this are considered zero fee for transaction creation (default: %s)",
                                                            CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MINFEE)), false, OptionsCategory::WALLET);
    gArgs.AddArg("-paytxfee=<amt>", strprintf("Fee (in %s/kB) to add to transactions you send (default: %s)",
                                                            CURRENCY_UNIT, FormatMoney(CFeeRate{DEFAULT_PAY_TX_FEE}.GetFeePerK())), false, OptionsCategory::WALLET);
    gArgs.AddArg("-rescan", "Rescan the block chain for missing wallet transactions on startup", false, OptionsCategory::WALLET);
    gArgs.AddArg("-salvagewallet", "Attempt to recover private keys from a corrupt wallet on startup", false, OptionsCategory::WALLET);
    gArgs.AddArg("-spendzeroconfchange", strprintf("Spend unconfirmed change when sending transactions (default: %u)", DEFAULT_SPEND_ZEROCONF_CHANGE), false, OptionsCategory::WALLET);
    gArgs.AddArg("-txconfirmtarget=<n>", strprintf("If paytxfee is not set, include enough fee so transactions begin confirmation on average within n blocks (default: %u)", DEFAULT_TX_CONFIRM_TARGET), false, OptionsCategory::WALLET);
    gArgs.AddArg("-upgradewallet", "Upgrade wallet to latest format on startup", false, OptionsCategory::WALLET);
    gArgs.AddArg("-wallet=<path>", "Specify wallet database path. Can be specified multiple times to load multiple wallets. Path is interpreted relative to <walletdir> if it is not absolute, and will be created if it does not exist (as a directory containing a wallet.dat file and log files). For backwards compatibility this will also accept names of existing data files in <walletdir>.)", false, OptionsCategory::WALLET);
    gArgs.AddArg("-walletbroadcast",  strprintf("Make the wallet broadcast transactions (default: %u)", DEFAULT_WALLETBROADCAST), false, OptionsCategory::WALLET);