In no particular order...
No, not tired. Enjoying the graphics
This should probably come before everything else: We don't worry about image duplication, at least on the technical side. Simutrans recognizes identical sprites when loading paks, so memory consumption is proportional to unique graphic count. This means you can have two 1:1 trees, only differing in one age graphics, without any performance hit at all. Trees also have DistributionWeight, so it is easy to compensate for the imbalanced probabilities that might result from such practice. So... you can simply draw variants of ages or seasons as you please! (To be fair, we poor humans are somewhat worse at managing redundancy organized in this way, so try to stick to whole rows/columns in the hypothetical table )
Saplings didn't exactly work for me either, and I can even rationalize why
Perhaps your local variant grows as white from young age, but over here, the young trees have at first brown, then maybe grayish bark and turn into the characteristic w/b perhaps at some 3-5m. Otherwise, I liked the idea. If you managed to pull this off, you could have two versions with & without these.
As to dead trees, hard to say. You are probably right that they do not add to aesthetics... much. On the other hand, I remember actually liking seeing some of the trees dead. A forest with these just felt more real, somehow. No offense meant to your elvish side!
Theoretically, you could have two variants of a species, and for their last ages one dying and one "rare", while still reusing the first 3 ages... I don't want to imply that you should paint so many, though! Just thinking aloud
Anyway, tree life cycle period lengths are 1:1:1:6:3, so focusing on adult #1 = win
Err, it is tomorrow already? In the end, don't mind my rambling that much. I'm probably too sleep-deprived...