Mac CPU PECI is at 104°C when encoding videos and doing heavy work
No, there's no need to worry.

The CPU will automatically shutdown before reaching a critical temperature that could damage the CPU. There you do not need to worry about imminent heat damage.

In addition to automatic shutdown, throttling is almost always also used before the critical temperature is reached to ensure that you avoid the shutdown.

Therefore - if you're not experiencing that the computer shuts down unexpectedly under high load, and your computer is functioning at reasonably normal speeds - there's really nothing to worry about.

I won't focus to much on the specific 105 degrees measurement. First of all the measurement software you're using might not be fully precise (PECI temperatures are always negative and an offset from critical temperature - not an absolute measurement like yours, so the software you're using must be doing a conversion), secondly the specific TjunctionMax for your CPU is calibrated individually for each CPU in the factory. So your TjunctionMax might not be 105 degrees, but rather 106 degrees or something else.