I altered my plan a bit, used the ruler designer to create a Hellenic count of Anfa. Thus, my starting character (y767) is a tribal Hellenic. Plan is to conquer what I need as a tribal and then change to Merchant Republic. If possible, I'm aiming to eventually have my capital in Alexandria. I remember something about merchant republic capitals though, is it so you can't change your capital at all?
also, according to the Wiki Hellenism doesn't limit succession laws even if unreformed. But as a tribal that doesn't help. So if I want to get rid of Gavelkind, I have to reform Hellenism anyway, right?
Unreformed pagans cannot become merchant republics. You could presumably game that by converting, switching to MR, and converting back again... Merchant republic succession should trump unreformed pagan mechanics, but I wouldn't want to risk it myself. So, yeah, plan on reforming first. That won't allow you to play super-tall, though, since the holy sites are spread out over Greece, Italy, and Egypt. (But maybe that part of the plan was altered, too?)
I'm also not 100% certain that MR mechanics will override unreformed paganism - but I think it's very likely. (If a pagan reforms the faith, then becomes an MR, there will still be many unreformed pagans hanging around. Eventually one of them might become doge.) All in all, I'm not too worried about this.
The only concern I have about temporary conversion in order to become an MR is that the 4 auto-generated patrician houses will be of your temporary religion (not unreformed hellenism). If @Tur8oed has access to the console, he could fix this immediately by painstakingly forcing the religion of every member of the 4 patricians' dynasties immediately after forming the MR. Otherwise, he'll probably have some difficulty for a while.
For that reason alone, I'd suggest waiting to form an MR until after you've reformed hellenism.
On the other point about staying tall - you could always keep 3-5 holy sites in your personal demesne, and grant independence to everything else.