Firstly, you must be in World Tier IV of Diablo 4, as these unique features are exclusive to the game's highest difficulty level. While some unique D4 Items can be found in World Tier III, these six items are not among them. Next, you need to defeat enemies at least level 85, as any enemy below level 85 cannot drop these unique items, and unlike other unique items, these cannot be found by opening chests or destroying objects.
Therefore, the best way to cultivate these super rare unique items is to focus on content that offers the most unique items each time you play the game. If you're seeking these unique items, RPGStash suggests farming Nightmare Dungeons.
Although other avenues can often get you unique items faster, such as Helltides, it's usually because you can obtain them as rewards for completing events and unlocking chests. Given that these six super rare items are locked behind defeating enemies, this approach becomes outdated.
When running Nightmare Dungeons, make sure you're at a level that spawns enemies at level 85, as it won't scale to your level. Hence, ensure your Nightmare Sigil is at least level 32 before embarking on the adventure.
By following these strategies, you'll have a better chance of acquiring these super rare Uber Unique items in Diablo 4. Good luck on your journey through Sanctuary!