Depends how the Achievement reward system is structured. If it's going to dish out rewards in a structure similar to existing diaries for each tier completion and not on a per task basis then I would recommend boss slayer tickets. They would be consumed on use to not make them OP and keeps them limited. It should require having "like a boss" unlocked via a slayer master and by redeeming it would guarantee your next slayer task will be a boss task. The boss task received would still be randomized as to not cause unnecessary crowding by players selecting preferred bosses to farm. At the end of the day rewards for achievements should not be something crazy op or necessary for an account but more a bonus to incentivize people to try out combat achievements and not make it a necessary grind.
What happens when you've done all the achievements, would it be possible to get more tickets or do you work with what you have and enjoy the temporary benefit of them?