But 'Indirect Level Sync', every method I have ever seen suggested, it seems that someone can come up with a way to abuse it within an hour or two.
The first thing that usually comes to mind when I was thinking about this previously is a layering of the boss room and mechanisms or some npcs casting shit that only happens if you have low lvl players in your raid/party (would probably have to be quest-based).
The boss itself would adjust its difficulty to the amount of high lvl players present, but the overall difficulty requirement would stay the same. The low lvls players would still have to deal with their mechanics at the same high lvl as the big bois. This would also be a great initiation into the game's high lvl pve stuff.
Low lvl players would be on the outer layer and would be fighting their own mobs (and potentially mini-bosses) that were agroed by those players' presence. The mechanism/npc would have to be protected and/or used at crucial times of the raid to allow the high lvl players to progress through their fight.
All of this would obviously have to be very precisely balanced at any and all potential adjustments to the ratio between high and low lvl players. And if designed correctly, you could even have reverse difficulty where the high lvl part is somewhat easy, while the low lvl part is hardcore. Or it could even swap throughout the boss stages. In other words, there's a ton of ways you could design it, but it would obviously require a ton of work and testing to get it all just right.
The boss rewards would be the same as if the full high lvl raid killed it and the low lvls would get their rewards through their quest (and maybe some cool cosmetic for achieving this feat).
This would also perfectly fit my preferred method of ability acquisition (that is "you get all your base abilities by lvl25), because people would start experiencing how their full set of abilities plays asap.