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.
LOCK(wallet.cs_wallet);
    if (wallet.IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER)) {
        
        PartiallySignedTransaction psbtx(mtx);
        
        
        bool complete;
        wallet.FillPSBT(psbtx, complete, SIGHASH_ALL, false , true );
        const TransactionError err = wallet.FillPSBT(psbtx, complete, SIGHASH_ALL, true , false  );
        if (err != TransactionError::OK) return false;
        complete = FinalizeAndExtractPSBT(psbtx, mtx);
        return complete;
    } else {
        return wallet.SignTransaction(mtx);
    }
}
Result CommitTransaction(CWallet& wallet, const uint256& txid, CMutableTransaction&& mtx, std::vector<bilingual_str>& errors, uint256& bumped_txid)
{