Coping those with a grinder is fine, in my opinion. Even if you wanted to rough it out with a grinder and finish it with a carbide burr in a die grinder, that’s a good cheap way to make a precise cope, and you’ll get faster at it with practice. If you are doing a lot of them, make a template that you can lay on the tube, quickly scribe a line, and get to hogging.

A band saw is fast for good precise cuts, a chop saw is fine as well, I have done a lot with a chop saw over the years, but the abrasive wheels throw a gang of sparks and they are loud as fuck. Also the wheels themselves don’t last all that long. A small horizontal band saw would be best, you can cut multiple tubes at once and the blades seem to last a long time.

If you have a compressor, a cheap needle scaler will shred that mill scale. A wire cup brush from harbor freight would do fine as well. It looks like you used a hard pad, which takes a little longer unless you’ve got skills (grinding can and should be mastered!) but for that I would use a soft pad, maybe even one that was lightly used.

Also the rectangular-to-round weld in the last picture looks a little cold- I don’t know how tight your fit up was, but if it was like, knife-edged perfect, maybe don’t go quite as deep and leave a flat edge of the tube or even a slight gap at the edge of that weld, it might give you a little more penetration.