Hello and welcome to Piano Society!
I was in the same boat a while back. I don’t think you can expect to sit down and play through some of your old pieces and think that you should automatically be able to play them. It takes time to get your ‘fingers’ back. At least it did with me. The desire to come back to piano was so strong in me, that I began practicing hours every day, and thankfully my sight-reading improved significantly and I was able to learn a lot of new music. I still have to work very hard when I have something new to learn, but it’s a labor of love. You will get there too! You just have to play every day. I think scales and arpeggios are good. I tried Hanon, but didn’t feel it was worth my time. A few years ago I started taking piano lessons again from a very good teacher, and it was the best investment I ever made (besides buying myself a new piano….haha)!
Best wishes with your piano playing. I hope you let us know how you are progressing. And maybe you can make some recordings for us too!