Heya,
You should be ok with the 18-55 on a tripod for the night sky. At 18mm on your APS-C, you can expose for about 17 seconds before trails become obvious. So if you stop down to F4 at 18mm, it will help a little with the shape of stars and overall sharpness, likely shooting at ISO 3200 as a starting point for 16 seconds. Look at your histogram to have some data in the left 1/4th of the histogram since you won't have light pollution. Focus with live view at 10x magnification on a bright star. Then just sit back, enjoy, and do a few exposures while you night gaze.
If you get into it, I would instead point you towards a tracker before worrying about lenses as a tracker mount will give you a ton of flexibility for exposure time and relieve your camera of ISO level needs. The Star Adventurer is a good starting point ($300). You can use any lens for the milky way then, instead of trying to get the fastest lens you can and still needing a lot of ISO.
The 18-55 will be great for general landscape, it's plenty good at F8 and F11.
I actually wouldn't even take the 50 STM unless you feel the need to do portraits and want soft backgrounds, but I would think in that environment you'd want full depth of field, so if you're stopping down, you might as well have just used the 18-55 for simplicity and convenience.
Very best,
