You can certainly feature a dungeon which you plan to take the heroes from, say, 1st to 6th level. And then have it have 6 levels. And have the treasure suitable for a 6th level party distributed over the 6th dungeon level.I was thinking dungeon level, but yeah. Even though I’ve accepted that will happen some due to running a sandbox, I’d still want to make sure I wasn’t going too far from the system’s expectations.
In this case, you're making level into a big central concept, so I would run that with the RAW level-to-proficiency rule.
If you plan on using the proficiency without level variant (to "bind" accuracy, as it were) I recommend sticking to the core loot guidelines. Or running the "no magic loot" variant, etc. Otherwise you're just creating a problem for yourself. While I totally see the allure in "the level 4 Ogres guard level 4 loot, while the level 7 drakes guard level 7 loot" that inherently trusts level as a meaningful concept. And running proficiency without level essentially takes that away.
My players would totally make plans for that drake nest already at level 3 just because the variant lets them, and because running around with severely overleveled loot is the minmax move; which essentially justifies the GMG advice I just said wasn't needed