I'm going to the crag with some friends in a few days and I plan to lead the first route, clip into a quickdraw on one of the anchor bolts and build a top rope anchor on the bolts. I've been practising the anchor styles that I could use and I am in the situation below. I know that I could simply unclip the quickdraw and slowly lower myself onto the anchor but I don't want to fall onto the anchor, even though it would certainly handle the fall. How can I slowly and properly transfer the load from the quickdraw to the top rop anchor? Are there any interesting ways to do this or should i just unclip the draw and get the belayer to take and lower?
You are clearly overthinking this. Even if your are hanging freely, you should be able to take away your weight from the rope by pulling on gear and unclip the quickdraw (there is nothing that can go wrong, you are on belay). Still there are different ways to do this in a controlled fashion, in practice I only use the following.
If there is one solid piece of protection (a functional bolt certainly qualifies and anything that you trust with your weight, so anything that you intend for a belay, should qualify) is to use a sling (or something equivalent) to attach yourself to it right at the beginning (instead of your quickdraw). Be aware that at this moment you do not call out "belay" or whatever other command you use to signify that you are safe! You are hanging on a single piece of pro, so your belayer should continue to have you on belay. After that just build the anchor, tell the belayer to tighten after you finished and you unclip your sling. This of course works also at the end of the scenario you described with using a quickdraw.
You could also just use a sling to step into to help you with getting the load off the rope. I am sure one can come up with more ways (you could even build an improvised pulley), but that would be a real overkill. Keep it simply, keep it save.
I wouldn't do this the way you propose. If I understand you correctly, you want to clip the top bolt, then have your belayer hold your weight while you set up a top anchor, and you want to know how to unclip your quickdraw after and load the top anchor.
My first question would be, why aren't you using a personal anchor?
My second question would be, why wouldn't you just clip both bolts with draws and use that as your top anchor?
Finally, to answer your question, the best way to do this would be to use a Purcell Prusik in place of the quickdraw:
Use the purcell prusik as a personal anchor, clip the top bolt with the prussic fully or mostly collapsed, set up your top anchor, have your belayer take up the slack in the rope, then slowly extend the Purcell Prusik to load the anchor.