The levels displayed on the star chart aren't "recommendations" and have little to nothing to do with the level of your own Warframes. They're just an arbitrary measure of the power level of the enemies in that node.Like, I have an Excalibur frame is that completely leveled, with fully leveled weapons and everything. So I play with my frames that aren't leveled, so that I can make some progress on them (which, as I recall, you gotta do to progress in the game). And yet, despite those frames being near the recommended levels, I am unable to make progress of any sort. Does the game want me just to stick to the frame that I've completely maxed out, or what? Are you supposed to stick to your leveled frame to fill in the star chart and just grind out previous missions to level up new frames?
You talk about leveling up your Warframes but don't mention anything about the mods you've installed on them. Do you have the survivability mods like Redirection, Steel Fiber, and (especially) Vitality installed? Are they levelled up? Those mods make a far greater difference than any intrinsic bonus you'll get from levelling up a Warframe.
Alternatively, get yourself a Rhino and forget about having to even think about how much damage you're taking for pretty much the entire starchart.