So let's look at the ways you offered:
Eureka is the only way to make progress on the Stormblood relic. It would make no sense to tie a second relic to it (not to mention it would create an even larger mess of items in a player's bags than the first one did - I've still got my chocobo saddlebag filled with crystals, etc. I'll never use and keep forgetting to discard).
PotD is already an option for at least one stage of the Bozja weapon (I never finished mine so I don't know if it could be used for multiple stages). If you're thinking of Deep Dungeon in general, Eureka Orthos will reward Causality if you're using a level 90 job. There's an option for you.
Ocean fishing is a gathering activity. Gathering and crafting now have their own relic tools to work on. It makes little sense to use them to earn a relic weapon (even if it had been done in the past) when players now have the relic tools to get.
PvP - has PvP ever been a method for getting a relic weapon completed? I don't PvP so I would automatically tune it out if it was offered as an option. That said, I personally wouldn't have objections if PvP was an option as long as players had non-PvP options to choose.
Tribal quests - unfortunately, Arkasodara rewards Comedy when done on a level 90 job. Even if it was Causality instead of Comedy, it would end up replacing Variant as your least efficient option so it surprises me you'd bring it up.
Bozja - like Eureka, it already has a relic weapon heavily tied to it. It would make no sense to add a second.
To me, the only one out of your list that would make sense to add as an option is PvP. Adding current expansion FATEs could also be an option if they can solve the problems seen with the FATE farming in the early stages of the Resistance weapons.
In general, it makes more sense to me that level cap content would be the preferred source when working on relics. Lower level content frequently had its own role in other relics.
If players want to pursue the most efficient option simply because it's the most efficient instead of because they actually enjoy the content, that's their choice. Nothing says you have to choose the same. Getting a relic isn't a race.