Here are some details on how it went.

Firstly, Visa in inbox for Pakistan
We haven’t met anyone yet who did the same process weirdly. This is different to the Tourist Visa, where you need to go to a Pakistani embassy. The Visa in Inbox is a sort of Visa on Arrival and it can be done when passing the border at Taftan (for residents of some countries only, the list is on the official website).

We filled in the questionnaire a week before crossing and only uploaded a screenshot of our booking.com booking for a hotel in Islamabad instead of getting a letter of invitation.
As soon as we submitted the form and paid the $35 fee, we received confirmation emails and were able to download an ETA or Electronic Travel Authorisation.

Upon arrival at the border, on the Iranian side and specifically Hamid (King of Taftan border who offers help and assists you through the border crossing for no fee), they wanted to make sure that we would get the via from Pakistan before they stamped us out of Iran, specifically because they pointed out that on our ETAs it mentioned showing them ‘at the Airport’. It seemed like they hadn’t seen an ETA before and that the Pakistani system hasn’t been updated for land borders. Hamid printed them out and had them brought over to the Pakistani side. We waited a fair bit but then received an email with our visas. On the site, the status of our application went from ‘In process’ to ‘completed’. Hamid printed out our visas and sent us on our way.
We didn’t need to go to an embassy or wait for approval in Zahedan, we only waited a little at the border. Probably the easiest visa of our trip!

Next, the border crossing. It was long, hot (end of June) but once we were stamped out of Iran, it only took a few hours to be stamped into Pakistan. We exchanged dollars for rupees when approached by a man after getting our passports and carnets stamped. Then we went to the gate for the final writing down of information and photos (twice) and waited for a policeman to escort us the 2 minutes to the police station. We had been told by Hamid that we wouldn’t get an escort towards Quetta that day so didn’t bother leaving Zahedan too early and knew to expect an overnighter in the police station.
The set up there is primitive. If you can, bring clean drinking water, they only have filtered and it’s not great. The toilet in the tourist room was disgusting and leaked, there was only carpet to sleep on so if you are equipped for camping you will be comfortable enough if you can bear the heat (no AC). The fixer can go to the shop and get you basic provisions (get water!).

We left Taftan at 7.30am and arrived in Quetta at 00.30.
This seems weird given the distance but it included 2.5hrs stranded on the side of the road when one of the Levies’ pickups broke down and all the time spent at the many checkpoints + a few hundred kilometres doing 30 kph in the dark despite our protests that we could ride faster.
Note that we were in a convoy consisting of us on 2 motorbikes, a truck that could only do 80-90 kph, and the levies’ pick-up that the travellers without vehicles were carried around in (changed 15 or so times).
Also note that the levies don’t communicate amongst themselves so one set will have no idea if you last stopped for water 10 kms or 100 kms away.
They also don’t grasp the fact that unlike the other travellers, motorcyclists can’t ride and drink/eat at the same time so we actually need a stop for lunch and a stop for dinner that doesn’t last just 3 minutes. We had to refuse to get back on the bikes on two occasions so we could eat.

Pack food and drink - we struggled to find food that would be good for eating in the go on the Iranian side so be sure to plan in advance. Supermarkets in Zahedan weren’t all that good for this purpose and we regretted not spending more time earlier finding a good place to buy snacks.
Expect that all the water you do pack and that you haven’t gone through during the border crossing and the overnighter in Taftan will be near boiling temperature after being cooked in the sun for an hour (at least it was for us in end of June). We had temps between 30 degrees (at midnight in Quetta) and 47 degrees (most of the day). Sometimes you can get cold water at the stops but don’t count on them as there is no logic to when the levies let you stop.
Our water was nearly all finished by the time we arrived at the Bloomstar hotel.
We paid 4,000 rupees for a double and stayed two nights because getting the NOC takes some time and the next place to sleep if going to Islamabad is 500km away.

Getting the NOC - the police picked us up from the hotel at 10.30am and took us to the Home Office. We were then granted our NOCs (the process took an hour and a half) and the letters were given to our guards. We told them we needed an ATM and SIM cards. They took us to an ATM - so far, so good.
Then we were transferred a couple of times before being taken to a road with the phone companies’’ shops. We were in the Jazz shop for hours, getting everyone’s phone set up because the shop assistants didn’t know how to configure the phones properly. We think we were just unlucky.

Having left the hotel 6 hours previously, we were desperate to get back and get food, but despite our pleas to be taken to the hotel, we were transferred a couple of times and taken to the police station where we put our foot down and demanded to be taken to the hotel because we were just going to be hanging about there while they took photocopies of our NOCs.
I wish I could say that the 8 minute drive from the police station to the hotel took less than 10 minutes but alas is was another 40 minutes before we finally arrived.

In summary, once you get to Quetta, you are at the mercy of your guards and I recommend bringing water and snacks even if you think you are just going to be out for a couple of hours to get your NOC. Once it had been drafted, ask for it or they will keep it with them meaning you are not free. Even once you have it, you are not actually free to go wherever you want, stay in the hotel and order food in.