If I create an RBF transaction using the Qt GUI but specify my own custom change address, bumpfee doesn't work because it claims it can't find the change address:

    "Transaction does not have a change output"

This is because none of the outputs satisfy

    pWallet->IsChange(...)

I see there's a "TODO" commented in `CWallet::IsChange()` in `src/wallet/wallet.cpp`:

    // TODO: fix handling of 'change' outputs. The assumption is that any
    // payment to a script that is ours, but is not in the address book
    // is change.

I tend to put all my addresses in the address book whether I'm going to use them as change or not. I guess if the wallet was capable of creating its own SegWit change addresses I'd stop using custom change addresses, but currently it always seems to send change to a legacy address if I don't manually specify a change address.
CAmount& old_fee, CAmount& new_fee, CMutableTransaction& mtx, bool require_mine, const std::vector<CTxOut>& outputs)
{
    
    CCoinControl new_coin_control(coin_control);
    LOCK(wallet.cs_wallet);
    errors.clear();
    auto it = wallet.mapWallet.find(txid);
    if (it == wallet.mapWallet.end()) {
        errors.push_back(Untranslated("Invalid or non-wallet transaction id"));
        return Result::INVALID_ADDRESS_OR_KEY;
    }
    const CWalletTx& wtx = it->second;
    
    
    std::map<COutPoint, Coin> coins;
    CAmount input_value = 0;
    std::vector<CTxOut> spent_outputs;
    for (const CTxIn& txin : wtx.tx->vin) {
        coins[txin.prevout]; 
    }
    wallet.chain().findCoins(coins);
    for (const CTxIn& txin : wtx.tx->vin) {