Steam store is a bit wonky on versions, probably for marketing reasons. It looks like what's listed as Tamriel Unlimited (for $29.99) is the base game + the Morrowind expansion. You'll notice it says "Buy The Elder Scrolls Online - Morrowind" but when you add it to your cart it shows as Tamriel Unlimited.
The $19.99 Morrowind (DLC) is the upgrade price for people who previously owned the base game and want access to the new areas and classes.
Digital collectors editions are what you'd expect. Aesthetic gear, mounts, etc. If that has value to you, then go for it.
ESO plus membership is their subscription, but not needed to play. It gives you perks like bonus XP, a monthly allowance of crowns (their in-game real money currency,) which you can use to buy DLC packs, mounts, aesthetic stuff from the crown store
I don't have any of the extra stuff (including Morrowind and the new classes) and it doesn't feel like I'm missing out on anything. The world is fairly massive and the quests now scale with your level, so there's a ton of content that you never "outgrow." Maybe some of the others who are a bit higher level can speak to the benefits?