Batman: Beyond is in some universe where there are also superheroes but it's not exactly in the same timeline as the Justice League animated series. They did make a comic series for it, but it still seems to be some alternate timeline.

I think retired!Bruce is kinda hilarious. He's that grumpy old man who wants to kick kids off his lawn, but he can actually physically do it himself if he wants.

Have you watched Batman Beyond: Return of the Joker? Or that Batman Beyond animated short during the Batman 75th anniv celebration?