Alrighty! I'll split this into a couple sections: first, video production tips!
Audio quality is super important. Video quality is important too, of course: high resolution and framerate while recording will go a long way, as will nice-looking visual elements like overlays and end slates. But audio can make or break your LP channel. If your voice is hard to hear or your mic makes you unpleasant to listen to, a lot of people won't stick around. LPers kinda have to be low-level audiophiles. If you look at the most famous LPers out there, a lot of them have some impressive recording spaces for this very reason. If your hearing makes this difficult, see if you can get a hand from a friend who has an ear for audio! And always prioritize your voice over the in-game audio when editing/mixing in post! Being able to hear the game is important, but if viewers can't hear and understand you, then the whole experience falls apart.
Practice being entertaining! Being a Let's Player is deceptively difficult, way more than most people assume. Talking and constantly being entertaining while also concentrating on the game you're playing is super hard. But being charismatic is maybe the single most important key to Let's Play success. Let's Play channels run on personality. Your viewers could easily play any of these games themselves, or watch any other YouTuber play them, but they came to your channel because they want to feel like they're hanging out with you. Being a Let's Player often means being an entertainer, even if you're not forcing a goofy or screechy persona. So keep practicing! Try to engage the facecam when you can. Don't go silent for too long unless it's appropriate for the moment in the game. Heck, take an improv class if you really want to pursue this stuff hardcore. This is a performance skill just like acting, so keep working that muscle.
Analyze other Let's Players for ideas! Pick a few channels that you really like and want to emulate, and then carefully study their work. What sort of visual elements do they add? Are their videos long or short? Do they edit their videos to trim out the boring parts? How do they start each episode? What sort of info do they put at the end? Learn from those who are already succeeding in this scene. If you don't know how to do a thing they're doing, that's a perfect opportunity to start researching! The more stuff you learn how to do, the better you can make your own videos.
Ok now for some general YouTube tips:
Make your thumbnails eye-catching. You want them to entice. This is why you see so many channels using bright colors, starburst background patterns and faces featured prominently; it's the kind of stuff that catches the eye. Of course, also try to make the thumbnail give an accurate idea of what the video contains so people who are looking for your kind of content recognize that your video is what they were looking for. Featuring your own face isn't a bad idea, and featuring art/logos from the game you're playing is probably a good idea too. There's no secret to success here, just keep experimenting and improving your designs as you go. The more clicks you can draw, the more your channel will grow, so this is a pretty important component of each video's success.
Release videos as regularly as possible. Daily, if you can. It allows you to become a daily part of your subscribers' lives, part of their routine. It's a great way to build and maintain a regular audience.
Try playing new or popular releases now and then. When a game first comes out, lots of people are going to be eager to see that game played on YouTube and Twitch. Hopping on that train can be a great way to draw in some new viewers. Of course, you don't have to do that all the time. Again, personality is the real key to success; your regular viewers will likely enjoy seeing you play anything. But if you only play older games that you never got around to in your Steam queue, you may have a hard time building a large audience as quickly.
There's no single shortcut to channel growth. Success on YouTube is equal parts luck and dedication. Release videos regularly, strive to make your content more and more entertaining, work to become a more charismatic personality. Doing cross-overs with other popular YouTubers can help, but it won't guarantee a fast-track to success. Sub 4 Subs almost certainly won't fast track you either. Your best bet is to just be dedicated, strive for quality and do what you can to stand out. And play to your strengths. Are you funny? Charismatic? Maybe you're an especially skilled player? Or maybe you have something else unique to bring to the table? The more you can stand out in a positive way as a personality, the better.
I think my single biggest tip would be this: if being a professional YouTube personality is your goal, throw yourself into that shit. Trying to become a famous YouTube personality these days is like trying to be a famous rock star or famous hollywood actor. It's harder than ever: the scene is flooded now and extremely competitive. Success is far from guaranteed. All the famous streamers and Let's Players you see out there making a living doing this? They work their asses off. Easy as this career looks from the outside, it takes an enormous amount of time and dedication.
So if you want that goal, commit yourself to it fully! Do everything you can to be the absolute best you can be. Study the platform, study your peers, watch your channel analytics and see what you can learn from the patterns. Try to make content that you'd want to watch. If you keep making great videos and releasing them consistently, the audience will slowly find you and the snowball will start rolling.