The heat isn't too hard to deal with, there are solutions (including the weird ones like water cooling cases https://www.ebay.com/itm/175003613243 ). Simply adding airflow tends to get you pretty far by itself, even. Device test labs are a common-enough thing and handle running phones like servers at scale without too much difficulty.

But there's more nasty lurking problems in this area, like that phones aren't designed to be continuously powered. They will naively try to keep the battery at or near 100% charge, which will destroy it relatively quickly, and not uncommonly in the "it's bulging and increasingly likely to burst into flames" variety. 2 years is considered a "decent run" for things like device test labs as a result (see eg the FAQ on https://github.com/DeviceFarmer/stf )