Next, I started studying a little bit because my classes will get back obegin again in a few days, and I have to be prepared ( a lot of things to remember hahaha).

Today, I woke up later than I had wanted to.

Since this is referring to something further in the past and isn't describing something sequential, we need to use the past perfect.

I had planned to go to the gym at 8 a.m, but I had to change to my plans, though.

You can also replace the comma with a semicolon or a period, but I think this transition makes more sense.
Separating independent clauses with a comma can sometimes be justified stylistically, but it's rare and usually there is a strong sense of parallel structure between the clauses.

I got up and made my first meal, which was a mixture of eggs, tapioca flour, and chicken.

This is grammatical, but "tapioca flour" sounds odd as a breakfast food and suggests instead that you are describing a list of ingredients that you used to make a certain dish, especially because you also used "mixture."

Then, when I finished, it was about noon, and I decided to have lunch;: a simple meal with rice, chicken, and some vegetables.

The semicolon should be used to separate independent clauses or to supplement commas in a list. Here the colon is more appropriate.