The RPi will have zero issue with heat, it will throttle the processor if it starts to become too hot. Even throttled it will run the OSPi just fine. If you really want to reduce power consumption you could under clock the processor quite a bit. Remember the RPi uses an processor that is used in cell phones. If you haven’t had to add a heatsink to your phone adding one to the RPi isn’t needed either. If fact some of the fancy colored, anodized aluminum heatsinks can actually cause overheating issues.
By underclocking my RPi3 and turning off HDMI and Bluetooth I was able to get it to run at just about 1.5A, much less than the 2.5A it requires with the base settings. I did it for a weather tracking project that automatically looks for and takes pictures of reported storms near my house. It gets some great shots of storm clouds, but I digress. It sounds like you know your way around the Pi so if you want to avoid the soldering iron just go for underclocking and you’ll be fine.