Personally, I find that it's generally best to simply expand the charts above 10th and balance the rest with bonus feats.
Looking it over, I would do it something like this:
11 DR 11/adamantine
12 DR 12/adamantine, +1 caster level
13 DR 13/adamantine, bonus feat
14 DR 14/adamantine, +1 caster level
15 DR 15/adamantine
16 DR 16/adamantine, +1 caster level, bonus feat
17 DR 17/adamantine
18 DR 18/adamantine, +1 caster level
19 DR 19/adamantine, bonus feat
20 DR 20/adamantine, +1 caster level
None of their other abilities advance regularly (although starmetal rigor is a close one, if the values stayed the same), so I don't think anything else from the first 10 levels would increase- so I'd peg feats about every 3 levels, I think. I'd also make some class-specific feats that would enhance the character's GSA abilities.
If you want to include a starmetal vigor increase I'd put it at another +/-1 at 13th and 16th, and another +2/-1 at 19th and 22nd, then +1/-1 25th and 28th, etc. In this case I'd drop bonus feats to 1/4.
