They've never been "very difficult" if players bothered to pay attention to the mechanics. You move out of the AoE, you interrupt if it can be interrupted, etc.
The real issue is how quickly they die now. A Shadowbringers S rank could be pulled as soon as it spawns but as long as the relay goes out right away and players immediately drop what they're doing to get there, they have time to be part of the kill. That's not true with the older ones so that's what needs to change.
We've already been told that the amount of work required to reprogram hunts for level sync doesn't make it feasible. What if instead of level sync, the hunt marks were given a buff that allowed them to absorb damage taken over X amount from a single attack?
Throwing random numbers out as an example, let's say the largest amount of damage from a single attack by a level 50 player (without a critical hit) is 2000. Level 50 hunt marks are then given a buff that absorbs damage above 2500 (to allow some room for crit damage by level 50 players but not so much that every attack by a higher level player becomes an effective crit). Dancer has a 100k hit with Technical Finish? S rank takes 2500 damage (97500 damage absorbed). If a level 60 player could deal up to 10k in a single attack then the level 60 hunt marks absorb anything over 10k, etc.
Hunt marks are back to taking more time to die, allowing more players to arrive and participate. It doesn't prevent level cap players from trying to solo lower level hunt marks. They still get to use their level cap rotations. They're still going to retain their higher health pools and their greater chance to dodge attacks that would otherwise hit but it will take them much longer to kill the marks. It leaves it up to their discretion on whether they want to still do it on their own but take a long time or if they want to relay to bring in more players to help to speed up the kill.