I
love the rent-an-in-house-designer concept - it solves a big mental block I have had with the 99designs style approach.
However I get the feeling your price point may be too low - there is a lot of value a designer can add, not just in "logos and stationary", and that can mean a lot of work - I guess you just have to suck it and see.
Which takes me to tiers - it's a lot easier to have the "you are now in the platinum tier" conversation than the "stop asking me to do a full time gig on 200 bucks"
As it is so hard to guess how much work, and you want to get away from
looking like it's piece work, can I suggest offering streams of work. Bear with me
Logos and stationary are one basic stream of work.
But the long term gigs will probably come from two classes of company - mid sized firms wanting to get in-house designers to revamp the existing art work to be more consistent, and I strongly suspect internal enterprise teams wanting to build distinctive internal brands
That to my mind gives three streams - the try-me-out level of logos and stationary, the internal branding and the wider revamp. Each feels like increasing amounts of complex work. but each feels like a distinct market, and with levels inside
Edit
I guess the internal brand one is most exciting to me - give developers a pattern library for their app, and keep adding to it as they add features or work. A bootstrap/react pattern library that you can retheme would probably go long way?