I think it might be possible to compromise and keep everything within a contained range. Off the top of my head, PC levels 5-9 seem like a good range. The various adventures or villains or whatever could all be built around those levels and if the 5th level PCs decided on the 9th level adventure, it would certainly be tough but not instadeath. Likewise, a 9th level party could enjoy kicking the crap out of a 5th level adventure, but a few bad turns of the dice could still create tension on occasion.
So, to a large degree, power in 5e is linear.
A level 5 PC is roughly half as powerful as a level 10, who is half as powerful as a level 20.
This is just ballpark. And here, "half as" means "if you took a challenging encounter for the level 5 PC and doubled the number of foes, it remains challenging for the level 10".
Using the encounter building rules:
1 CR 7 vs party of 3 level 5 is 2900 xp, between hard and deadly
2 CR 7 vs party of 3 level 10 is 8700 xp, a touch above deadly
4 CR 7 vs party of 3 level 20 is 23200 xp, between hard and medium
By literally just scaling monster count with level and ignoring all of the DMG math and XP per PC and encounter sized multipliers, I have scaled an encounter from level 5 to level 20.
The point is that by the time you are in 5-10 region, the amount you can scale up is very bounded.
Monsters with insane CR are beatable if you have a good strategic advantage. And even at level 20, monsters you'd have to carefully defeat at level 5-10 remain a threat if you deal with them casually.
Or... you can leave the monsters alone. Completely alone.
Make the bosses CR 15-20 or so. Level 1-4 you cannot hope to defeat them. Level 5-9 you can only hope to defeat them if you (a) get them alone, (b) have a huge strategic advantage, or (c) bring along an army. They are
Strategic foes, not combat encounters.
By level 10, a CR 17 foe that you tactically isolate can be defeated in a single fight. It will drain you, and you better be fresh, and you better not be unlucky. But you can do it.
Those same foes by level 15 are single encounters. Or you can fight them with allies. You no longer have to be strategic or tactical - they have to be tactical to deal with you!
You can drop the BBEGs CR to 10 or so to make this transition happen earlier. Leave CR 17+ for forces of nature, like the tarrasque, or a dragon living in a volcano not interested in your petty politics.
Now, a CR 10 boss is unbeatable level 1-4. By level 5, if you brought a strong ally and isolate them you could beat them. By level 10, they need allies to keep themselves safe from you.
You can weave this into the game expectations; maybe there is a one-off ally that can help the PCs with the first BBEG, but is defeated/leaves later on. The bad guys can go from thinking they are invulnerable, to defending themselves better as the PCs level up.
Finally, the bad guys plan might involve gaining personal power. And the success of that plan (despite the PCs getting in the way) gives the bad guys a CR bump. Like, one bad guy might turn from a CR 10 necromancer to a CR 17 lich.