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.
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return UniValue::VNULL;
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    if (!request.params[0].isNull() && !request.params[0].get_str().empty()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Dummy value must be set to \"\"");
    }
    UniValue sendTo = request.params[1].get_obj();
    mapValue_t mapValue;
    if (!request.params[3].isNull() && !request.params[3].get_str().empty())
        mapValue["comment"] = request.params[3].get_str();
    UniValue subtractFeeFromAmount(UniValue::VARR);
    if (!request.params[4].isNull())
        subtractFeeFromAmount = request.params[4].get_array();
    CCoinControl coin_control;
    if (!request.params[5].isNull()) {
        coin_control.m_signal_bip125_rbf = request.params[5].get_bool();
    }
    SetFeeEstimateMode(*pwallet, coin_control, request.params[6], request.params[7], request.params[8], false);
    std::vector<CRecipient> recipients;
    ParseRecipients(sendTo, subtractFeeFromAmount, recipients);
    const bool verbose{request.params[9].isNull() ? false : request.params[9].get_bool()};
    return SendMoney(*pwallet, coin_control, recipients, std::move(mapValue), verbose);
},
    };
}
RPCHelpMan settxfee()