First of all, I'm not a user of your UI work. I gather my addons and mod them, adding some few own-build addons if necessary. I'm therefore giving an objective view on the problem, as I'm not interested in it.
I kinda understand how you feel about leaving your work "un-complete" (even if it looks quite complete already, there are still things that you've thought it would be nice to add, etc...). But the thing is that for the member that would subscribe, even if it looks like a small amount, it's comparable to the fee to play the game. I mean that if you're paying a monthly 70% of WoW's fee for your UI, you somehow consider that either WoW is underpriced and could really be more expensive, or that 35% of your entertainment comes from your UI.
I have myself a lot of fun changing my UI and coding random stuff, but would many people agree? Especially as, whatever you do, there will still be ways to download your work from other sites, as members might give this UI to a friend of them that asks for it, which might once again give it to someone else etc. I'm not sure if you were planing to restrict the DL to members, or only the support, but I think that restricting the DL is impossible and restricting the support might be of low interest for the subscribers.
So what I would advise is to give a life-time membership instead, for maybe $15-$20 (I have no real idea how many people would like that though). More people would probably subscribe then. It would still be the same kind of people as those who made donations (I mean, if people just want to steal the addon, even with no knowledge in anything, they can install emule or a torrent client), but it would be more fair for them.
I'm not too sure what you'll think about that, but that's what I would go for if I were you (unless there's something I missed, of course).
And...
Good luck whatever you do there and IRL anyway
Edit:
The poll has no "I don't use Mazzle UI even if I wouldn't push it in a fire just for getting attention" ^^