The most important limitations are already said. Especially to be aware of battery and how to deal with that is a good thing and I guess you will have no issue as you described. What you should be aware of is the 3GB RAM on standard L5 and 4GB on L5 Liberty (~1GB used for system) as well as 32GB or 128GB storage. Especially for the standard device you should also buy a micro SD to put all your media on.
Not all apps are available for ARM, but most are. In general it shouldn’t be a problem, especially not for foss apps, but maybe for proprietary ones. So things like Signal and such non Linux native apps can be tricky to run or even (currently) impossible. The biggest issue you may run into is the supported screen size and/or touch support for such apps. Therefor you may want to give a look into this thread. Non-fitting may can be used with little adjustments. For example I’m using KeePassXC and that has no good phone UI, but with little adjustments it’s at least usable (but not very comfortable).
The camera app is not state of the art, but photos and videos can be made. Sometimes you need to adjust parameter manually (via GUI) to make the photo look good. I think we will get further improvements over time, but that requires a lot of work. Also the camera-API is not fully implemented yet, so other apps have no access. That can be worked around with an USB-webcam if needed. But that API will come.
By the way, I was speaking more about limitations since you asked for them. I just want to mention that it also has advantages over iOS and Android. For example you also can run other systems like Mobian or PostmarketOS. But I think you get the idea of the system. I hope I could answer some questions.