Hi - we were on this cruise too and also had a lovely time.
Responding to some of your observations and sharing ours, our cabin steward also only came in the morning and was to be found servicing the suites down the corridor from about 5pm. She said we could find her there if we needed anything. She was very obliging. We actually preferred not having an evening service, as it meant we didn't have to rush to tidy the cabin before we went off to dinner. Two choccies arrived with the morning make-up service and the Horizon was put on the bed every evening.
Although we just had shampoo and shower gel in largish containers on the wall in the shower, our daughter asked for conditioner and got it, no problem.
Re the app, I managed to use ours this time, but it did depend on where on the ship we were. I liked being able to see my account on the app. Re booking dining, we were Freedom Dining and sometimes used the app, but sometimes went down early and queued up at the desk for a table for four. This worked well too. If we were later, we were given a buzzer and went for a drink in the nearby Spinnaker Bar and took the glass in with us if we were called before finishing.
We were really lucky with our table service - we tended to get in the same area of the restaurant each time and had a great waiter. Although we also noticed the fewer choices on the menus, other than on the last day, we had no problem finding dishes to suit us. Must say that I couldn't fault any of our meals - or temperature of the food, in fact we really enjoyed every meal.
We had booked four shows before boarding, but were never asked our cabin number, but there were always plenty of seats available though I had wondered, like you, how they would deal with it if over-subscribed. That never happened to us though. We used to go into the theatre about 8 10 pm for the 8 30pm performance and there were spaces in all our performances, even in the full audience ones.
I did find it frustrating however that the films were shown in the tiny cinema, so we never managed to get in there on the sea days - and were not able to view them in our cabins like you can do on some ships.
It was great to be onboard Arcadia again after so long and seeing things coming back to normal bit by bit...