enhancement bonuses go up to +8
insightful bonuses go up to +3 and stack with the other types
exceptional bonuses are +1 and stack with the other types
An item called litany of the dead gives profane +1 bonus that stacks with the other types
Tomes go up to +5
There is a guild buff that can add +2
There are Yugoloth potions that give +2
Reasonable depends on what you have access to... adv. packs, plat and TP all come into play here. I would say bare minimum sustainable would be a +7 item and a +3 tome and +2 from guild buffs (even if you have to ask for invites) If you have The Shroud raid you can craft a +2 insightful/+1 exceptional int weapon too. If you have TP or a fortune in plat you can buy a +4 tome maybe.