This is not poor. With 2 active sims that is actually quite good. Just think for a minute why the flagships of the top companies almost never get dual sim option (at least in Europe and America). If you need dual sim there are some downs to that.
I got a root. After that, I changed the minimum frequencies of the processor (they are high in the drain, no matter what mode we choose). Chosen by a conservative governor. Removed all Google software. I set the refresh rate to 60 Hz. Disabled AOD. Sim one is 3g, the other is 2g. But I still get the same 5-6 hours. I have a suspicion that the reason for the fast discharge is the radio module.
So I have Zenfone 8 since June and battery life is OK, I can get through a day. Saying that I am a pretty heavy user as I had to charge my pixel 3 twice in a day, while now when my father is using it, battery lasts for 2 days with 4-5h screen on time.
Here is my accubattery estimates. They have gone from 6h 10min to 6h 48m in the last week, so maybe the adaptive battery is affecting this.
I think 5-6hrs with moderate usage is alright
First of all it has an 888 and flagship socs in general are known to have an higher battery consumption plus the higher refresh rate (and depends if you use 5g all the time or if you get a better reception in your area and other usage)
You can optimise some stuff in Asus autostart manager,disable aod (which consumes quite a bit of battery) and some other stuff and get slightly better battery life though
But Asus might optimise battery further with otas though
So, having used a few days now it has suddenly got much better. I'm off charge since 7am and am writing at 4pm and I'm at 80%. Not massive usage - 1 hour screen time with a little Pokémon go in between which seems pretty decent to me. I've disabled 5g, have screen on auto refresh rate and running latest firmware. I do wonder if low signal strength causes disproportionate power drain, the office saw me down to 30% by mid day work virtually no usage at all.