7 inches is nuts, a lot of other small cnc’s only have a 3 inch Z. You should really bring that down to 3 or under. Your bit only has .5-1.5″ cutting ability anyway, you should always put your material to be cut as high as possible. If you are only cutting 4mm deep only leave 6-7mm room. Cleaner more precise cuts. Half the distance twice the speed. Think of it this way, most 3D printers only have a 8 inch z and you are slamming around a 4lb spindle+the drag from its cord.
If your allthread has a rough spot take, it out and chuck it in a drill, lube up the rod and run it back and forth a few times until you feel the rough spot go away.
I feel 25-35 is pretty fast, sure faster is possible, but I don’t think I have made any videos faster than 20mm/s, I go slow and just very the depth to keep cuts real nice with minimal finishing work.
You machine sounds good your expectations just seem a little high. I can make the machine do more but, I am focused on making it easy to use for everyone, not just bleeding edge fast for perfectly adjusted machines.