I've made my own Black Walnut flooring. It's 3/4" thick and 4.5" wide with random lengths mostly in the 2-4' long range. I will be tongue and grooving it all. How about the butt joints? Should I be spline jointing the ends? Glue them without a spline? Glue with a spline? Thanks in advance for the advice!

Yes unless you want them possibly sticking up in the future.

If you don't spline the butt-joints they will move from vibration while sanding.
You could glue the butt-joints if not using a paper type of moisture barrier.
If you are using the moisture barrier you can cut out a small section for each joint and only glue the ends with a flexible type of glue like sub-floor adhesive.
You can nail or staple one end of the joint even though it has a square end and glue the other end.

If you are milling your own floor you should put a grove on the left side and a tongue on the right. This way you will not need spline. If reversing direction you should always glue and nail spline. 5" and over is recommended to glue and nail to subfloor.

putting a tongue and groove on the ends is a whole other process than on the edges of the boards. It also uses up more of your flooring.
Using a router bit to make two grooves on the ends, then using a spline to keep the ends secure will be the best that you could do.
Factory made end matching will usually have a sloppy joint so the boards can go together easily.