I'm afraid that you may never completely rid your plants of the thrips, but you can certainly hope to keep them under control. To do so, you will likely have to use a variety of methods.

I'd certainly apply the Safer's product again, as long as it is listed for houseplants (and indoor use). You should also try insecticidal soap (Safer is just one of many brand names). Neem oil is listed as a decent thrip control, too. Switch out your treatments on a regular basis....never use them all in one fell swoop.

Whatever you decide to use, make sure that you read the label thoroughly and follow the instructions. Never use anything on indoor plants that does not expressly come with those specific instructions that it is safe to do so.

Does any of this help at all?

I do have insecticidal soap and neem oil. I know I wasn't having any success with the insecticidal soap on its own, which is why I switched to the Endall. I don't want to use that indoors though, even if it is supposed to be safe. I've read conflicting reports about pyrethrin toxicity.
It's discouraging to think I can't get rid of them though.

It can be very difficult to totally eradicate a very persistent and resilient pest. I'd aim for the kind of control that needs attention three or four times a month....maybe less!

You are right in being cautious about the pyrethrin. It can cause problems for a pretty large percentage of the population. Is this product even labeled for indoor use? That is a very strict guideline to follow.