Stone is so competitive that there isn't much margin to be made in it by anyone unless they are a direct importer and fabricator. I have wholesale negotiating power, and some jobs end up being less than 10%, which doesn't really cover my time to deal with the paperwork and handholding for the client. But, that's what the market demands for entry level granite. Where we do well, is with our Cambria fabricator. It appeals to a less cost conscious client, and we can get a good enough wholesale price to get market rate on it and still have 15%. But, we have national negotiating power for that relationship.
With only a couple of kitchens a year, the hassle of you being the responsible party for that aspect probably isn't worth it to you other than as a convenience factor for your client as a "one stop shopping". If I were you, doing the volume that you're doing, I'd try to fold the stone price in with the whole job price. and not call out it's price in the contract at all. If you can't do that because of market pressures, then I'd find a relationship with someone who could do the fabricating well and that you can trust to be stand up, and just put the barest minimum markup on it for your time ordering it, and just mark up other material for the job higher to make up for the low margin on the stone.
Or keep doing what you're doing now and avoid the headaches.