I can't recommend Maldev Academy enough. It has been an amazing resource, to get into malware development. Keep in mind, however, that malware development is pretty difficult topic. You will have to eventually use WinAPI and syscalls, so learning about that even outside of malware development will help you a lot.

For example, try looking into how to execute a shellcode in memory - allocate memory as RWX, copy some data and then execute it. Try executing it in a different process, or in a different thread of another process. That's the core of malware development you'll probably eventually have to do anyway. Manually calling syscalls is also a skill that you'll need, if you want to get into EDR avoidance.

Also, look into IoCs and what kind of different stuff can be used to detect the malware. Syscall hooks, signatures, AMSI, and syslog are all things that are being watched and analyze to detect malware, and knowing what exactly is your program logging and where is one of the most important and difficult skills you can get.

There probably are a lot resources for these two skills, and they are an important foundation for malware developemnt, so I'd suggest researching that. You'll probably not get much from looking at other malware, because it tends to be really low-level, and obfuscated, exactly to avoid the IoCs I've mentioned above. Implementing the malware behavior after that is the easier part.

Another good resource to look into are C2s and communication, for example Mythic C2 has some interresting stuff.

And I really recommend joining the Bloodhound slack. Throughout my cybersecurity carreer as a Red Teamer, the community has helped me a lot and I've learned amazing stuff just by lurking.