I'll come at this a few different ways:

1) I would get over the idea that a hero mage is tough and that your goal is to power to hero; they can be tough, but my experience is that 9 times out of 10, if you can't win a fight with a midlevel invoker, you can't win a fight with a hero invoker either. Ditto transmuter or conjurer either.

Don't power to hero and then try to figure the class out; figure the class out and the rest falls into place. This also helps you with ranking despair -- people won't group with you? Go kill them.

2) You're ending up with marginal groups because probably you're a marginal groupmate. 90% of the midlevel invokers I've ever grouped with are worse than useless. They're spamming stoneshatter at the stupid mob that's bashing me to death. Short of getting lucky or riding the Fort Leveling Train to hero, seek to improve your value as a groupmate -- level with useful spells that you also don't have at 70%, learn to lead and coordinate groups so you're useful as a group leader rather than as the pity third, and put a ton of energy into a vibrant, well RPed character that people will WANT to group with.

I will take a cool character with me to level no matter how terrible they are, but bland neutral invoker #289784 is getting skipped for someone useful.