Some food for thought based on my experience growing with
OF and their trio bottle nutes.
Your plant looks underwatered and underfed mostly, I agree with the mg def but that I think is just the first sign of it being underfed. I ran into it alot when I started and yours looks about the same as mine did. When I was growing in 3 gals I would have to water daily during flowering and pretty much every other day by the 3rd week. That will be less if you're in a larger pot. I also had to start adding in tiger bloom(bloom nutes) around week 3 as well. I feed it 1/4 strength nutes every other watering. Although I watered till about 10% runoff so I didn't have salt buildup. A bit different than most people but it worked for me. As others have said lifting the pot for weight is a good way to tell if it needs water or not. Only thing I added to my soil was a bit of perlite. Hope that helps!