I had an idea on how to make training facilities that slow down over time, representing diminishing returns, or "you can't teach an old horse new tricks"
Unfortunately it doesn't seem to work, because multiple ship training abilities on the same facility don't work; it seems to be "one ability per planet", not "one facility per planet" is effective... what I did is, I made one training ability that trains at 1% per turn up to 6%, one that trains at the same rate but up to 10%, and another that goes up to 12%. That way, for the first 2 turns, you'd be training at 3% a turn, for the second 2 turns it would be 2%, and for the final 2 turns it would be 1%. I know it didn't work but I thought I'd bring it up in case it sparked any ideas...