I got it to work at a minimum acceptable level.
I had this put together for a coreXY machine I a building... this was made on the drill press but now that I have a milling machine I'm going to redo the x-axis (with a beefier linear rail as well).
No gear slippage but the door doesn't open all the way because the bar is in the way. I tried tying the string to the plate without the bar but it doesn't open enough.
I think it might be the belt though, it's sort of stretchy and I think a lot of the movement is getting lost in belt-stretch.
Thanks for the responses so far and still open to suggestions 