I think that the players should get the twilight forest when they don't have any problem dealing with the taint. Aka, they can make the etheral bloom, AND/OR they resist the taint with their sheer defense (thats why i suggested an netherstar). You don't want to give players the Twilight Forest to late, since a good chunk of it was made for vanilla players in iron armor. But if you want to make it endgame, might aswell make it need an archmage bloodorb.
Speaking of the Taint, i would suggest that - if it is possible - that the disabled taint monsters - like the taintswarm - spawn again after a certain period of time, like 20 ingame days - a purely tainted region only spawns tainted creepers, which are mostly harmless after getting some good armor.