Soldered RAM doesn't run hotter.

No meaningful difference in thermal performance between soldered and socketed RAM.

The heat RAM generates is trivial compared to the CPU or GPU. A few watts either way. Soldered modules might even run slightly cooler because there’s no socket resistance, but it’s not something you’ll ever measure or feel. Your laptop’s fans are not spinning up because of the DIMM slots.

If you’re worried about soldered RAM, worry about upgradability, not temperature. That’s the real trade-off: you’re stuck with whatever capacity you bought.

Don’t overthink the thermals.