I have very little knowledge on the upper end of the lake that you mentioned. On the lower end it has been my experience that the best smallmouth spawn bite can be out on the main lake or 1/2 to 2/3 the way back in a creek. I guess that's really not narrowing down anything since that includes the majority of the lake water. Conditions will be the major factor that plays into the peak timing of the spawn. On that lake for smallmouth I would say they start late March if conditions are right. By mid April it should be in full swing and by early May it will be slowing down. The largemouth will be on schedule that is 2-3 weeks latter than the smallmouth. If smallmouth were my main target I would say early to mid April would be the best time for numbers and size. For largemouth mid April to early May would be my choice. Mother nature will always hold the trump card. Things can change in a hurry in the spring. I have seen snow in mid April and I have seen 80 degrees... hence the reason I say mother nature holds the trump card.
As you mentioned and I agree the peak of the smallmouth and largemouth spawn do not coincide with each other. With that being said I also feel as though all bass whether they are smallmouth, largemouth or spots have different timing within their species. I feel as though the older/big females go a lot more early and spawn a lot deeper than most people give them credit for. They are what I call the first round of spawner's. I feel as though the next wave is the middle age and young fish. They typically are more shallow and are the ones most seen by sight fishing. The early big females are so deep you will never see them. Then there is a third late group of fish. They are primarily the very young juvenile fish. Sometimes in the third wave there is a few of the bigger fish that did not have a successful spawn and they made more eggs and dropped them in an attempt to have a successful spawn. With that being said it is possible to have pre spawn, spawn and post spawn fish in the same stretch of water... the entire last paragraph is just an opinion. I have nothing to back up my claim other than the crazy thoughts that go through my head.
Anyways "typically" late March to mid April is my peak time for smallmouth on Lake Cumberland. They can be anywhere from 2/3 the way back in a creek or out on the main lake pending conditions. Largemouth Mid April to early May. The only constant is Conditions will Always be the factor that controls the timing and mother nature controls the conditions.