I was in the same situation as you not long ago. To get me back on track, this website was a very good start, it provides the very basic informations you need to know in order to practice efficiently.
If you want to get more serious, I highly recommend this series of ebook written by Graham Fitch, a concert pianist. It might be what you are looking for, because it seems like you want good fundamentals but practicing scales mindlessly won't get you anywhere, even if you are very dedicated to it.
Theses ressources provide insights about how theses technical exercices should be approached to give optimal results, aswell as the overall strategy one needs to get the best out of his practice.
If you are already experienced but you feel like you lack proper training it might be a good idea to learn about practicing efficiently before actualy practicing.
If you want to study harmony, this book has every thing you need to know inside, is very comprehensive and has exercices to help you integrate the concepts. It is basicaly a must have if you want to study theory on your own. You will get tones of free informations by browsing the internet, but theroy is not easy and you will get lost if you don't have a solid backup book... Or a teacher.
There are also sightreading websites like piano marvel which might interess you. But as far as I know, the only thing you need to do to improve at sight reading is to practice :)
Finaly, you don't mention ear training in your question, maybe because you already have a good ear.
If it is not the case, I suggest incorporating it into your practice, it is an essential skill to develop ! Plus there are many apps that make it fun and easy :)
Have fun playing the piano again !