The way Zygor's works is there is an addon called ZygorGuideViewer that is just a UI addition that displays whatever guide you pick (which doesn't have to exclusively be Zygor's Guides). Granted, it offers some other cool capabilities like auto accepting & turning in quests and selling greys, but these are features that other addons can do as well. ZygorGuideViewer works within the bounds of LUA, which makes the addon itself (and therefore its use) "legal".
I would totally recommend using Zygor's if you are leveling alts or don't care about the quests you're doing, because it offers a huge leveling advantage in terms of streamlining what you do. Just let the arrow tell you what to do and you never have to think about where you're going or what you're doing. When I use this addon (and the leveling guides) I definitely understand why he charges for it.
For Horde, Vol’Dun the area captures the feel of Battle for Azeroth is both the hardest and continues than other areas in the game. It can get quite frustrating in this area so better to go it early when you can utilize your old gear. The next two areas are highly dependent on personal opinion – Nazmir is filled with mobs and Zaldalar is one of the easiest zones.
Still, it’s terrible to see the new expansion fail so spectacularly at iterating on Legion’s success. It’s hard to believe the two expansions came from the same company. Legion made you feel powerful and unique. Battle for Azeroth grinds at you by taking away as much as it gives. Sure, you can have a shiny toy. Sometimes. If you don’t get too carried away.
-Island Expeditions- Island expeditions was one of the new features I was looking forward to but Blizzard no longer makes games that are ready when they are ready but rather makes games ready when the deadline comes. Thus, many of the new features are completely stripped down from what was expected and it's literally just a small scenario you enter with two other players to continue to grind out your Azerite neck piece.
EXPLORE TWO FABLED KINGDOMS – As a champion of the Horde, travel to Zandalar to persuade the trolls to lend their naval might to the war effort. As a defender of the Alliance, sail to the seafaring kingdom of Kul Tiras, home of Jaina Proudmoore, and rally its inhabitants to fight for your cause. Once you’ve successfully recruited one of these proud kingdoms to your faction, infiltrate the enemy’s newly claimed territory.
While it’s clear that Blizzard is eager not to spoil the future of Battle for Azeroth, it seems pretty unlikely that players who want to be pure evil will ever get their day to shine in World of Warcraft. Despite the decision being added into the game, the good guys will likely win and the threats to Azeroth will continue to roll on by, getting knocked out by players one at a time.
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.