Honestly you NEED godlike beings to challenge tier 4 parties. 5e characters already punch way above their weight class. Slaying gods is not my cup of tea, but I did really enjoy 4e's deity statblock presentation. "Here is Bahamut, the actual god. If you slay his corporeal form, you need to go on this whole quest to actually kill him for good, otherwise he'll be back in 50-100 years." Then again, 4e had their epic tier built into the game; I can't say how well it actually worked because I largely skipped 4e, but my experience with tier 4 5e combat is not stellar.
Modern DnD will become high-powered high-fantasy after tier 2. I imagine that those of us that gripe about it (hi there!) would rather be running something else but can't for various reasons- so we complain and try to hack it to be closer to what we want. (modern dnd = 3e-5e; older stuff had extra rules for crazy power too ofc, but not as core accessible as modern).
But heck, look where BG2 ended up, one of my favorite games ever. I know it's a video game, but you WERE out there planehopping and dealing with demigods.