Banner: There is actually nothing wrong with your banner, but I would consider putting more color/saturation on it and make the text appear more pop and stretch it a bit more for mobile viewing.
Thumbnails: Here again be sure that the text is more visible, best using as few words as possible, you just want it to look interesting and show what the video is about. The title takes care of the rest. Also instead of all of your thumbnails for a series appearing the same you could change it up a bit with another background picture. Still the same text and feel, so people can still see it is the same series.
Titles & description: So this goes for the description. Make it a bit more descriptive, it doesn't have to be a lot, but it really helps your SEO if you use a couple of words from the title in the description as well. Your titles is good for search, because you're very specific with which game, episode & whether it's a walkthrough/playthrough/let's play. For making it stand out more you could use a line of something funny happening in the video in the title as well.
Tags: Viewing this on my phone so can't see your tags, but some quick tips. Be sure to fill all 500 characters out, better using possible sentences people would search for instead of individual words. Programs to help with tags if you're having trouble is TubeBuddy, VidIQ & Freedom!'s own Hearthbeat (all for google chrome as add-on's)
The rest of your channel is really good and good luck growing your channel