Sounds like you need a stabilizer element. Which one is "form without substance" again?
Yin or Yang might work as a stabilizer yeah, and canon gives us Sage Chakra which definitively works to stabilize the Rasengan's various elemental forms.
I think I figured out why Fire/Lightning is less significant burns. Heat and Vibration - that's a microwave isn't it. It's not that the burns are less severe it is that they are evenly distributed throughout the body.
Which begs the question of the arrival of instant meal heating seals. Charge the seal, let sit for 3 minutes, hot meal without any fire or light, and less chakra spent than a sheep
Fire + Lightning is one of only 2 Elemental Chakra mixes that doesn't have an obvious established combination result (the other being Lightning + Wind). Given the elements involved, Fire + Lightning = Electromagnetic Radiation
does make sense.
It wouldn't be 'evenly distributed throughout the body' though because electromagnetic radiation does not work like that, despite certain microwave oven seller's claims to the contrary. It
would, however, be a different type of burn from fire and one that might
look like it was less severe, or possibly even be less severe depending on how the energy conversion works out: Brief exposure to microwave causes
extreme pain, but very little actual damage as the outer layers of dead skin absorb most of the initial energy. A normal EM-spectrum Rasengan might simply not last long enough to cause severe damage, and it probably also wastes a lot of its power on less useful wavelengths like X-Rays, Radio Waves and visible light, which would account for the less severe burns compared to Fire.
(On the other hand, if it
is an EM-spectrum Rasengan, then that would imply the possibility of creating a
Gamma Ray Rasengan, which would be...
very bad for anything living that got even
vaguely near it. Definitely gonna need to be wearing Knight Armor to survive using
that technique.)