What high level adventures with non-world-shaking stakes have you devised or played in? What ideas do you have for such adventures?
I know this isn't going to be much fun/useful of a response from me, but those few times I've played in a high-level adventure, I had the feeling that the world-end-threat (WET) was in fact attached to it because it's a trope. The reality is, NONE of those adventures needed to feature a WET to be high level, and by converse NONE of the WET stories need to be high-level.
In fact, being high level is mostly about being powerful in combat. The trope is, the high level BBEG wanting the WET can only be stopped by killing it in combat, and only high level PCs can do it. But if you look at real world history, how many BBEG who caused a WET were good at first person fighting? How many were defeated in hand-to-hand combat? Practically none. Their WET were caused by their immense political and military power, but they were probably equivalent to 1st-level PCs.
Another hint about how much level doesn't really matter, is that so many times those high-level WET stories in a RPG rely on something which is outside the rules: typically an ad-hoc artifact or ritual which will destroy the world/open a gate for invasion/unleash undead plague/whatever. That's because there is nothing within the rules which can really cause a WET (maybe only earthquake or meteor storm cast every day). Even Wish can't, unless the DM allows... an ad-hoc use beyond the rules!
So what is the technical reason why a 1st level BBEG can't have the same artifact or cast the same ritual, other than the artificial requirement that it must be high level because the adventure is high level?
My conclusion is that almost every story can work at every level (unless some challenge in the story requires a very specific spell, but even then in most cases you can modify this as it is often arbitrary), and what level of monsters you use is more of a gaming concern than a narrative one i.e. you use monsters of level X pretty much only because the PCs are level X.
For your purpose, try taking a few low level adventures and replace all monsters with high level ones, I believe in the vast majority of cases the story will make the same sense.