Keep up the work! I've taught a few adult students over the years, one of whom had gotten to grade 7 in elementary/high school, and quit piano for 20 years. He came back, and after a few years, was floating around Grade 8-9, dabbling in Grade 10 rep (RCM. This is equivalent to about Grade 7-8 ABRSM).
The best thing with piano is the amount of Repertoire you can choose from... there's a book I've had for like 9 years, and I was flipping through it yesterday, and there had been a piece in it I never looked at or listened to. Gave me a pretty awesome 2 hour activity when I came back from school.
Adult beginners who are driven can do really well. Some tend to overthink stuff (they are sometimes worse than young starters with the question "why?" haha). I also had a student, she was an older lady, probably in her late 50's, who took lessons with me for four years. She worked at a leisurely pace (she liked playing 10-15 pieces from each grade before moving on, and only did an exam for every second grade). She made it to Grade 5, and was honestly one of the best sight readers I've ever had for that level in the RCM system. She could honestly pick a piece, and have gone through quite a bit of it for the next week, took some notes on it, and had started writing fingerings.
She treated these pieces kind of like mini puzzles, where the payoff is "beautiful music". I love people with that mentality!! She was one of my favorite students, she took her time, and she enjoyed it. I unfortunately moved across the country for school haha, so she was moved to my old teacher (who is excellent). But I'm sure if she stick through it, she'll get to the upper grades, even if it takes her another 5 years.