I use 1 thread on 28 count, over one, and I couldn't imagine using 2 threads with a full cross on 25 count as HAED recommend. It would hurt my wrists too much, all that tugging. But going to 20 count might just be the thing.
On my miniature embroidery (40 count and 48 count) I use a thread conditioner called thread heaven, which is a lot like a wax. I coat my thread with it by running it across the little tiny box at least twice before I start stitching. Silk gauze is really tough fabric on floss. It really wears it out, but the Thread Heaven seems to stop the fraying, and I can stitch using a reasonable length of it too. It works somewhat on metallic threads too, up to a point.
I also don't stitch my darker HAEDs on white canvas. I use the colors, so if the main area is a lot of purple shades, and I don't want any light show through I start with a light to medium purple ground fabric. Then if I get any show though's at least they're not white. Space Traveller is quite a dark design, but I wouldn't go to Black. I reckon that's too much hard work on the eyes. I don't know what colors 20 count comes in, but you could always try using a washing machine dye if it only comes in white.
I think 2 threads on 20 ct using full crosses should suffice. However, I would highly suggest stitching a 10x10 practice square with the 2 threads, just to be certain that you like the coverage. My HEADs have been on 25 count Lugana using 2 over 1 tent stitches and I have been very happy with that.
For the best coverage of the fabric use the following:
11-count aida - 3 strands
14-count aida (or 28-count evenweave over 2 threads) - 2 or 3 strands
16-count aida - 2 strands
18-count aida - 1 strands
22-count hardanger - 1 strand
anything higher than 18 I use 1 strand