I don’t know the WiiM app with Roon Ready option but I suppose it doesn’t give you the Roon GUI, and I suppose you like the Roon GUI if you are thinking about the lifetime license.
There are some possible causes for this, some more expected and some not so. There are many existing threads about this, search the forum for ARC and battery.
If you run heavy DSP (MUSE), it will drain the battery faster. Nevertheless, it shouldn’t get excessively hot.
There are some unexpected causes, i.e., certain content blockers. Some of this was investigated here:
There are also improvements being prepared in Early Access testing, should make it into production in a week or so:
ARC can use the system streaming option of the phone, so AirPlay on iOS and Chromecast on Android. It’s also possible to attach a DAC by USB to the phone.