I'm AFB, but, IIRC, technically, you use only your racial HDs to calculate your caster level with a Sylph.
And, anyway, the Sylph has pretty good survivality at lower levels. yeah, she loses a bunch of levels, but Outsider HDs aren't bad, and, AFAIK, the Sylph has permanent greater invisibility and fly, which added to the spellcasting makes her pretty playable for a monster with LA 5.
If you have houseruled that she can count her class level as racial HDs for spellcasting, the best you could do is go sorcerer. Getting 2 sorecerer levels each level is awesome.
If you have houseruled that, but you can't surpass your ECL with sorcerer, then I suppose your build is acceptable. I'd have gone with something such as Mindbender 1, Ur-Priest 2, Prestige class X for more spellcasting plus DMM goodnes, though. Because spellcasting is the only protection you will ever need.