Questing is one of this game's strongest selling points, but it has always sorta struggled in giving players some kind of player agency or decision impact while playing. Often times the choices or dialogue options you can choose don't have any impact on...well anything. In previous expansions, you were able to "get away with" a form of player impact via the after quest party. Where at the end of the main story quest for the expansion, there is some kind of gathering. And if you did certain quests/decisions; you would see them at this private instance party. But I think the impact of that was exhausted after using it many times.

Now doing a singleplayer style quest with branching choices is hard to do in a mmorpg. Let alone questionable use of resources as the repeatablility of the content comes into question. So here are some suggestions to help increase player agency without dealing with the issue mentioned above. And these suggestions are often used by other games, so they have a corresponding "proof of concept".

• Companions - With this new dialogue system you've got going, you can easily incorporate this into companions. You can do something really simple. Have an internal tag that's invisible to the player at certain dialogue choices in quests. "Good, Neutral, Evil". Then go to your companion system and make it so the companions lose or gain rapport depending on those choices IF they're active with the player. So ok, Companion X likes outcomes that are "Good" and has no impact with "Neutral" and dislikes "Evil". So if the player has that player active and they choose "Evil", well then they get negative rapport. Some companions that will be flipped, where they like Evil and dislike good. So you can use that as a system to help create player impact and feeling like decisions matter. You can expand this if you ever do romance with companions. Where if you're romancing a companion and you pick these new romance options in the quests, they dislike that.
  
• Allies at the finale Fight - With this system, the player makes choices and while those choices don't "change" the road that you go down in the quest (avoiding branches), it will impact the final fight to a degree. For example, you decide to spare some NPC during one of the quests? Well in the final fight they come in and will apply a debuff to the boss to make the fight easier. You decide to defend/fight for an NPC/Group in one of the quests? Well they come in the final fight and will take out some of the adds/boss bodyguards. Or fight alongside you against the boss. You see this design used in games like Baldurs Gate 3.
  
• Horizontal Rewards - Outfits, houseguests, furniture, weapon skins, armor skins, mounts. You have tons of these for the gemstore, but they can also be powerful rewards to help the player feel like their "decision" mattered. Spared an NPC? They can become a houseguest as the main reward at the end. Sided with a specific faction? Get some furniture themed for that faction. These horizontal rewards can be very good. Its just important that you make sure that each "decision" that offers these, you get something for both decisions. So sparing that NPC? Doing so or not, they both should result in something that feels equal in value. So if sparing the NPC rewards that NPC as a houseguest. Not doing so maybe a different NPC houseguest. Or a collection of furniture.
  
• Nested Decisions - So with this system, the reward of making a decision during the quest is the ability to make a new decision. What I mean by this is say there's a group of NPCs and they're going to attack some town. You have a dialogue option that says "Don't do it, listen to me". Normally, this doesn't stop them. However, if previously during the quest you made a decisions where you helped them. Say in a previous quest in the chain. Well this time they will listen to you and not attack. Now there will be no new branches created. So maybe the decision will be choice 1 -> choice 2 -> choice 3: rewards new outfit. But if you don't make the right choice in all three than the nested chain breaks. Making the wrong choice 2 makes it so choice 3 never appears.
  
• Bonus Rewards - You see this in games like Guild wars 1 and 2. But their story missions often have bonus objectives/side objectives that you can do and give you extra rewards. Sometimes achievement points, horizontal rewards, more gold, more exp, etc. ESO I don't believe has this. Another simple one.
  
• After Quest/Event Party - Same system you're using now. Its a good addition, keep it. Just don't rely on it on its own like you've done previously.
  

If these past two seasonal quest updates are to show anything, its that relying on these quests to make a major impact in their current design is not going to work. The thieves guild questline especially was RIPE with opportunities for the above. But you did none of that. Siding with a specific NPC not having any impact on anything. Maybe it could've had an impact on the finale fight. You have all these side objectives for staying hidden, yet I don't think they had ANY IMPACT. From what I can tell, no bonus rewards or anything. Such wasted opportunity. And its ok if players don't unlock everything on one character. Make any rewards account bound and let them make an alt. The current system you have people completing the content within one weekend. Then they talk about how bored they are and how the content was a waste because of how quick it was. THIS IS YOUR CHANCE TO CHANGE THAT. Give them a reason to hop on an alt and play through it again.