I'll cover things again, so as to try to get a single, complete list, and because I think there are some additional points to mention. I make the split into two categories:
Buffing your own save DC:
Improving your wisdom score up to 20:
Ability score improvements
Feats: Resilient, Observant, Elven Accuracy (XGtE; Elves, half-elves)
Ioun stone of Insight
Improving your Wisdom score beyond 20:
Book of Exhalted Deeds (upto 24)
Tome of Understanding (up to the innate max of 30)
Deck of Many Things: Star (upto 24) (pull cards from the deck at your own risk)
Increasing your proficiency bonus
- Ioun stone of Mastery (+1)
Weakening the target's constitution save:
Negative modifier
Bane (1d4)
Wild Magic Sorcerer's Bend Luck (1d4)
Mind Sliver (1d4; TCoE), thanks to Jesse Quimby for pointing this out
Disadvantage:
Bestow Curse
Sickening Radiance (through granting exhaustion)
Staff of Withering
Claw of the Wyrm Rune's Wyrm Ward (Storm Kings Thunder; Consumes the item, requires 8h prep work on the area, and only works on dragons)
Replacing the roll through Wizard School of Divination's Portent