I've included a number of mockups in this post for potential thermal system QoL, as well as some of my suggestions to improve existing visual aids.
In the first three images, I've done a mockup of a "Local Thermal System Information" box, which would display a thermal system's basic information when mousing over it both in blueprint mode and build mode, including missing part warnings similar to the shipwide warnings in the bottom left of the screen. I've also greyed out thermal systems in the images when they contain no external dissipation sources. TCMs count as external dissipation in this case (and heat storage), but the missing part warnings are still shown.
The other big visual shortcoming I've noticed is with heat exchangers. Way too many times, I've seen newer players think they can dissipate heat by themselves, so they attach them directly to overclocked parts, only for them to overheat. I would suggest either making this situation not show up as a heat connection, or placing a ⚠️ symbol in the immediate area as a precaution.
I put a good bit of work into these mockups, I hope they communicate my ideas thoroughly enough :p