Assuming that you’ve overcome your struggles with Tajweed and are at an acceptable level where you can memorise with ease and barely any major mistakes, the subjects that you can combine with Quran depend on the things you need to learn and your interests.
You can start with a summary or short courses in the following subjects:
This subject can be divided into all that interests you; most of the Islamic works rely on the Prophetic narrations and traditions, so the earlier you familiarise yourself with his words, actions and the names of the frequent narrators, the easier it will become for you in the future to explore this field in depth if you wish to do so.
Who is Allah? What is He like? How do I know more about my Lord? The subject is also known as the subject of Tawheed where the Oneness of Allah is taught to the believers and the importance of steering clear of anything that will put the pure faith in danger. Starting from shorter courses and books again is a good idea as this field is also very broad and will need full-time and attention if you wish to pursue this subject.
Learning the basic jurisprudence is also a must for every Muslim. The topics covered here are divided into essentials that everyone will need to know like how to purify, pray, fast, give charity, rulings on marriage and divorce, and then optional subject like the business and financial transactions for those who have a business and deal with financial matters.
With fiqh, the knowledge of the Principles of Fiqh – Usool ul Fiqh- also helps in understanding the sources of our religion and why some rulings are given preference over others in different schools of thought.
4- Seerah/Islamic History and Biographies:
Reading the history of Islam, and especially the Seerah of Prophet Muhammad is very valuable because it gives believers a boost to keep moving forward with the right intentions. Seerah never takes up a lot of time but you have to choose the right books and start with shorter biographies before moving on to larger volumes.
This subject will require a post on its own as it is connected with the memorisation of Quran more than any other subject mentioned above.
As for the subjects like Tazkiyah, self-help, comparative religion studies are all optional unless you’ve a special interest and like to learn.