I think you could cut the tartness of the cranberries by adding some lactose at bottling. My fruit beer that use sour cherries, blackberries and raspberries, was to tart, and lacking that fruity flavor. A little bit of lactose (3/4lb), and it solved the problem.
In any case, it is probably too late to be brewing a Thanksgiving lager for this year. A lager needs a couple weeks of primary fermentation, before being lagered for at leat 3-4weeks. So it won't even be ready to be bottled for 5-6 weeks from whenever you brew it, and since Thanksgiving is in 6 weeks, you will have no time to let it condition and carbonate in the bottles. Of course, if you keg, you may be able to have it ready on time, but this may take some experimentation before you get the right balance.