I think it is different to say you are end-game from your point-of-view and from the game's point of view. In terms of content, and considering I don't know your game situation, I'd say you are probably end-game if you are clearing Epsilon and can craft T12's, specially since we are talking mostly of pre-T13 opinions. Now in terms of clearing the game, or "completing" it, you can still be far, but that doesn't mean it isn't end-game, since end-game isn't the game's end but the level at which the content starts to wane and you focus more on honing your skills(or in this case heroes) and perfecting gameplay. I'd say "actual end-game" is when you start having noting to do, but the end-game I'm talking about is when you can* think about* reaching the end of events, crafting the highest level blueprints, focusing on completing the book as a main goal, having the workers maxed etc. Now this is when you can think about it in a realistic way, not necessarily already being in the process of doing those things.. I'd say that is end-game content-wise.
As an example, über heroes is something we feel the need to have, but if you can max events and clear all dungeons with "good enough" heroes, then those heroes are end-game heroes even if some people have perfect ones that make yours look medium.
But in the end, this is a concept so we can't ever agree on an exact thing. To me the most important thing here is whether this event is balanced across all phases of the game, or as it has come to my attention, keep it late-game but make it more obvious it is that way.
Going away from the off-topic(which is my own fault), I'm not sure I can come up with an alternative or solution for what OP is suggesting. Maybe giving the respective energy for the donations would help, specially since it would proportionate the means for people to energy-rush their crafts? But it would also make it annoying to go away from that screen all the time.