How to get constant 5v power supply when using same power supply for motors & micro controllers
I would say you could use some larger capacitance next to nodemcu. Also, make sure the battery is OK with current spikes you want. Don't forget the more current you draw, the greater the voltage drop on the output due to internal resistance of the battery (battery is always a voltage source + resistor in series, that resistance varies from battery to battery a lot).
If your voltage converter doesn't allow you to have large output capacitance, which can be the case, I would probably try to put a beefy diode on buck converter's output in the direction of NodeMCU and then a large capacitor after the diode. If you care about voltage drop, maybe some beefy schottky. Some 1n5817/1n5818/1n5819 from ebay diode sets should do the trick, they have like 0.15V-0.2V voltage drop, which you probably won't care about(?). Try some smaller electrolytic capacitor first (make sure it's correctly polarized). And of course  your little decoupling cap must stay there, it plays a different functional role.