Transition to Pooping outside while living in a flat

I have a six month old Chihuahua which I have had since 10 weeks. The extra couple of weeks with the parents and the very attentive breeder did wonders for toilet training and we only had to reinforce use of the puppy pad when we got her via treats and praise. Since then she is very good at using the pad.

We are now attempting to transition to outside peeing and pooping. The peeing is going well. I am taking her outside every two hours and she usually goes within a few minutes of standing outside. She still has the pad available inside for the time being which she does still use.

However, in the whole time we have had her she has pooped outside once, and that was when she had a bit if a runny tummy. We praised her and gave treats on that occasion. But still the issue is that she will never poop outside. No matter how long we wait, how much we vocalise asking her to poop, she still just holds it until we get back inside.

I am wanting to move away from using the pad during the day at all, and she is only allowed to pee when she is outside (provided I can take her out every couple of hours) but this also means she has nowhere to poop. I fear taking the pad away and then telling her off if she poops on the floor might just lead to fear and trying to poop where we can't see her.

We don't have a garden so going outside does require us to leash her up and take her downstairs ourselves.

Any advice?