I’ve basically set up a working circuit using jumper wires and a breadboard. I want to convert to a more secure/(semi-)permanent setup without using a breadboard. What kind of connections should I use? I’m assuming directly soldering onto the J21 header isn’t recommended.
That’s a standard 0.1" header. Headers of all types of wire connection can be purchased inexpensively. Depending on what you are doing the part which tends to make the most difference is if the wires used are twisted pair (in the case of a balanced pair), and if the wires are shielded.