Hi Honest,
Although I have a smartphone I never use it for ‘serious’ videos. Yes, I know many folk make some first class videos using such phones, but having been in the video making scene (as a keen amateur) for many years I have always used what I would call a ‘proper’ video camera. However, you have what you have, so let's go from there and please don’t think I’m talking down to you.
The first question I would ask is ‘What sort of videos do you like taking?’ Just as a few ‘for instances’, are you interested in taking family videos of the children at play – indoors or out? Maybe holiday videos in exotic places? Team sports games, again indoors or out, the list is endless. As already suggested, having a decent tripod will improve your videos 100%. A tripod with a proper pan/tilt head will be the best.
However, in all the above ideas for videos, the use of a tripod, especially a substantial one, would be inconvenient at least. One of the great advantages of a smartphone camera is its portability. Take from the pocket and it’s ready to go.
So I would like to suggest putting in lots and lots of practice in holding your camera as steady as possible. Using the W/Angle whenever possible will give you steadier shots as opposed to telephoto where every little shake will show up in the video. For the moment, forget spending good money on extras, but spend much time learning to use what you have much better.
You ask about the focus of your camera and I can only say RTFM. (For those not sure of that meaning it’s Read The Flipping Manual). Re the rule of Thirds. There are so many YouTube videos explaining all about this sort of thing and there are many tutorials on YT to assist in getting better results from a phone camera. Viewing these is much better than any of us trying to explain in words. Making good videos is something that takes a lifetime to learn - and even then there will still be a lot more to learn.
Back to square one . . . Tell us the sort of subject matter you are keen on and we can discuss further.
With best wishes. Pete.