I'm playing on a Mac. I highly recommend Minion. Minion makes it easy to search for, download, install, update, and uninstall add-ons. Since Summerset is new, many add ons are getting updated right now, so it's good to open Minion and "update all" before starting the game up. That's what I do, anyway.

Some add ons that you might like include:

For looking up items in Guild Stores: Awesome Guild Store
(If you're looking for a specific item, use tamrieltradecentre.com, the add-on doesn't work well on Macs, so I just use the website.)
For displaying inventory pictorially: Inventory Grid View
For searching your inventory (even across alts): Inventory Insight
For locations of relics, etc: Map Pins

In addition to the addons mentioned already;

Quest Map to keep track of quests, although it hasn't been updated for Summerset yet.

For your inventory I'd recommend Auto Category. It's highly customisable and can make inventory management so much easier and tidier. It's my absolute favourite inventory mod. Advanced Filters is also great, and there's a lot of extra filter plugins for it too if you're looking for something even more specific.

If you're into fishing or going after Master Angler I recommend Votan's Fisherman and Rare Fish Tracker. You may also like Votan's Fish Fillet and Bait Counter as well.

To keep track of the available skillpoints, I'd also recommend Urich's Skill Point Finder. This addon tracks all available skillpoints in the game, and whether your character has collected them.

I'm a completionist too so I love things like this that help me keep track of my goals.
I'm also an addon-aholic and could probably find an addon for anything, so if you want anything more specific then just say and I'll see what I can find.