Nice! We at http://draftss.com had been thinking to do something similar for the startup and open source software community.
You seem to have a flat monthly fee structure - and at a fairly "high" price point. If I was "just" looking for a logo I could pay your monthly fee just once at somewhere like 99designs, so I might not take retainer. But if I had greater needs for design 250 a month would be a bargain and you are too low and you will be doing a lot of work for nearly zero.
How has it been working out?
Have you considered tiered pricing ?
It has been working out really great, We have launched around more than month back and we are at $3,325 in MRR. We are writing our journey here : Zero to $3,325 MRR in 30 days (http://draftss.com/blog/index.php/2018/06/18/zero-to-3325-mr...)
We are looking for a strong feedback on our pricing model. What do you mean about tiered pricing? Would love to know more :)
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?