A six week kit following the instructions spends roughly one week in primary and five weeks in carboy. For many of us, that same kit spends one week in primary and five MONTHS in carboy. So what point is yours at?
Winexpert makes two Washington Rieslings according to their web-site. Which one are you making?
The Selection Estate one includes a Suss Reserve pack to be added with the K-meta, sorbate, and clearing agents. That will sweeten it a bit.
The World Vineyard is described as having a "hint of residual sugar". Not sure if that is from a sweetening pack.
BTW, the bitterness may be caused by a failure to degas properly. I think that some people perceive CO2 in the wine as a bitter tatse.
Why dont you sweeten it with some simple syrup, thats what almost everyone does or yoiu can use Wine Conditioner which is basically the same thing but also contains some sorbate to prevent the product from fermenting in the bottle. Simple syrup consists of 2 cups of sugar added to 1 cup of boiling water until it is dissolved and then cooled and added to the wine tasting very frequently and stopping right before you think its perfect as the sweetness will come through a little bit more in time unlike adding oak to a wine where as you actually over oak it as it mellows into the wine with time. Just remember to taste often as you can keep adding it but you cant take it back out. The best way to do it is to adjust a 1 gallon jugs worth and then multiply what you did in that jug x five and add that to the other five gallons just in case you accidentally add to much you can just chuck it in and start again fresh which will obvioulsy need less this time to both.
I added some conditioner and some simple syrup to a VR Reisling kit I made last year. It was very nice. Probably too sweet but it has softened with age. I think a little sweetening is what you need. Take a few sample glasses and measure different amounts of syrup for each glass and see what you like. We found that the wine seemed to get sweeter after a few months in the bottle, so I would suggest to stop as soon as its sweet enough to be drinkable, in other words don't over sweeten. I have the SE Columbia Valley reisling aging now and it had a reserve pak.
I am just about ready to bottle, is it too late to add the simple syrup. When I add the syrup, do I need to remix/stir the wine. Should I rack it prior to adding the syrup. These are all questions that I have.
What I usually do is draw about half a glass with a wine theif and then add syrup in small amounts to the glass, then taste unti I get it where I like it.
If you have sediment in the carboy it will need to be racked, based on what I decided from my tasting/adjusting I add some syrup to the clean carboy before racking. After racking I will wait at least a few days before sampeling again. If at this point it still is not sweet enough I add more syrup, stir, wait a few more days, sample.......Once I decide I like it I usually wait at least a week and sample it one more time just to be sure.