thanks @Lugia for the response/recommendations!
the Flame Quad Cv Recorder looks like the exact kinda thing, even mentions in the manual that the:
Smoothing algorithms eliminate the so-called 'stair-step effect' and ensure smooth playback even at very slow speeds
That said, it does look like it might be tricky to record fixed length automation without sacrificing a gate track for specifically start/stopping the record. I think thats where the voltage block is more appealing... downside being its limited to 16 steps and has quite a lot of crossover with what the digitakt would be used for anyway for the size of it.
I tried out just slewing the recorded envelope from the digitakt through the maths. Couldnt get as large a range wthout having the stair step effect occur, but worked pretty well!
With this in mind ive revised what I think want to buld next:
feedback and advice on what I should add/replace again really appreciated!