how can I be supportive as a friend-without being recycled as a partner? is it even possible?
Being recycled won't happen unless you dive back into what you've been trying so hard to get out of. He can't "make" you do it if you are unwilling.
Are there specific patterns you are afraid of falling back into that you want help with?I don't want to say... *SO what?*. I have limited my comments to *good for you* and *I am glad you are going and I hope it helps* and the like.

Sounds like you do have the right idea!
It sounds like you accept that he has a lot of work to do, and may stick with this and get somewhere... . or may not get very far, and you aren't getting involved. Another good sign.
I am trying NOT to rub in how OK I feel. How well I am doing by myself.

Trying to be kind to him is good for both of you.
Is it *fair* to be supportive? As a concerned friend would be? I don't want to give him the wrong impression. Yet I don't want to be unkind either. he needs as much help as he can handle!
Yes it is fair.
Remember you can't control his impressions, feelings, or reactions.
If you are kind to him, you will feel better about yourself, so keep on doing it as much as you can. (I'm reminded of some personal crap I'm going through... . where it has become very apparent that I am more honest than kind if I have to choose. Others were more kind than honest, and over time that made a bigger mess and everybody involved now feels worse for the false "kindness"
Be ready to kindly modify your own actions in the future if the results don't match your good intentions.