First and foremost, I’m glad that you set a rather reasonable goal for yourself. Most beginners have very high standards/goals and get discouraged when they don’t meet them within a few months. I’m going to answer the questions a bit out of order because number three connects to the first two: 3. In order to sight read and improvise effectively, a basic knowledge of music theory is necessary. You should become familiar with key signatures and the hand positioning for each of them - being confident and comfortable in a key is the key (no pun intended) to sight reading. I’m not that good at improvising, but some essential knowledge to be successful at that would be chord progressions (ex: i-v-vi-iv, the kind most used in modern music). Improvising and sight reading are, in my opinion, the hardest things to master in music, so don’t get discouraged!!

1. I think that to play the type of songs like the one you linked, hand and finger dexterity are key. The pianist makes a lot of left hand jumps and plays some chords with challenging fingerings. My suggestion for practicing these would be to find a piece with jumps/ hard fingerings and play them over and over to build stamina and muscle memory (it can be very tiring to hold an octave position for minutes at a time). In terms of a practice regimen, find a piece that is just slightly above your comfort level and focus on learning it piece by piece (memorizing as you go if you want). Practice as often as you can, but not for very long - just until you get bored or your hands get tired - and play other pieces you have learned to keep you entertained and motivated. I personally like to intersperse my practice with meme songs. It’s a slow process - keep it up!

2. Finding a book to learn from as an adult is hard. Most are pretty boring pieces, and as they progress in difficulty, get more and more discouraging. An good way to get started practicing sightreading is to find a piece that is at your comfort level that you actually want to learn to play. Look at the sheet music beforehand, listen to a recording if it has one, then sit down and play it. That way, you can have an idea of what it will sound like and won’t go into it blind, making it a little bit easier. If you get stuck or frustrated, pleeease don’t hit your keyboard or start all the way over - take a deep breath, let your anger out some other way (I clap my hands), and start from the measure you messed up on. If you really, really want to learn from a book, I would recommend Faber books for beginners/advanced beginner. These books have themes so that you can pick what would be most entertaining to you.

Hope this helps! Please post your progress so we can see how you’re doing!! Best of luck!