I used an agent (Speedy Visa) more expensive, but they did everything. Yes, you are stuck without a passport for a while. I have renewed in Thailand before and the previous system allowed you to keep your passport whilst a new one was being processed. Personally, I don't think it is a good policy they have now to leave people stranded without the ability to travel. I was left hoping that no emergencies or whatever came up for 3 or 4 months which I was not entirely happy about.
I hope that helps.