LENR is simply dark frame subtraction, isn't it?
It is, but the key with LENR is they tend to be ideally matched in temperature to the preceeding light frame. That allows for more ideal calibration, without leaving remnant FPN behind. I recently did a LENR vs. master dark calibration comparison. The master dark, generated with frames ranging from 35C to 47C EXIF temp, usually left remnant FPN in the data. An example 35C master-calibrated dark:
The LENR calibrated frames, even at higher temps, were perfectly flat:
Now, technically speaking, if you could match the lights to the darks ideally, then it doesn't really matter when you take them. If you have the ability to get darks every 2C for a wide range of temperatures that your camera may operate in, and create separate masters for each temp, then you could calibrate batches of lights each with a matching dark, and potentially get better results. That tends to be extremely tedious, extremely time consuming, and even with light to dark matching and library management tools, there is still one critical flaw: the EXIF temp of Canon cameras (most other DSLRs don't even have a recorded temperature, or if they do it's even farther removed from the sensor) is from the DSP, not the sensor! There can actually be fairly large discrepancies between true sensor temp and the EXIF temp, enough to have as much as a doubling/halving of measured dark current for the same recorded EXIF temp. So...matching by recorded temperature will often have an undefined outcome.
LENR, on the other hand, will usually match within a fraction of a degree. It may cost you more imaging time...however, it's worth testing to figure out if the remnant FPN you'll have to deal with if you get more integration time will actually give you a better image or not. Sometimes, half the time with LENR will actually deliver more pleasing results.