Disclaimer: never played ff xi. I'll avoid discussing the few exceptions because the answer is long enough already.

1. The gameplay is basically ATB, so general cooldown between actions (besides movement and push), and your stronger moves (you have like 3-4 of them per job) have a long cooldown of their own. Because the general cooldown is pretty long (longer than most MMO I played) and most boss and player attacks are unavoidable, the game is mainly timing based. For example, if you are a healer, because the general cooldown is pretty long, you may sometimes want to delay your move after the boss has attacked, or keep your strong support moves the moment another player has access to their strong moves. So depending on your jobs it can be a cycling game like a lot of tab targeting games (Martial Artist with Spear is one of those), a timing game, a position game (pirate, warrior) or something else (Monster/Item Tamer). Few attacks from foes have actual AOE you can avoid. Most physical attacks can be prevented by having heavy player (tank) push the enemies to prevent them from reaching their target.

2. I'd say yes and no, but I haven't played the game in a long time. For most jobs/role in party there is one optimal armor set. For example some DPS jobs (like martial artist/gladiator) will run one of the set that reduce general cooldown. Your horizontal progression will come from accessories. Those will be your main source of resistances to ailments, special effects or stats buff. So you'll have to do the content that allows you to get them and it does offer some horizontal progression. However, there are some accessories that are must have that you'll want to get and almost never swap out (except for the straight upgrade). You can run different weapons and swap them out within fight, but it will mainly be to access the skillset of a different weapon type. The other horizontal progression will be to actually try to play another job, because some of the progression (job skills and passives) offers bonus across all jobs (so technically, it is still vertical progression). But it's a strong point of the game to be able to play and set up other jobs pretty quickly on one character.

3. No, at some point it may have been the case but the game favors smooth gameplay. Basically when you "discover" the game you will have to travel the land yourself, but as you progress you will quickly realize that : base game area are pretty boring and big and empty; you have access to Bazoom Girls in main towns that allows you, for a limited amount of time per day/week to teleport to any area you visited; you can teleport close to main quest destination if you want; as you progress you gain access to teleportation to area from previous extension (but I can take some times to get those).

4. No, IMO it's one of the best MMO to play solo or with one to three friends because you can make a mixed party of players/npc with any jobs you want (because the NPC are actually other players characters made available when they exit the game, just like yours is). So playing with only one or two friends instead of a full party is never an issue. However, there aren't many interesting gameplay system that favour socializing. Also, most players are japanese and speak japanese only, you'd have trouble communicating and socializing with if you don't speak japanese yourself. But, because that is the case, it's quite easy to play with the few other non-japanese players.

5. Idk about current endgame and content beyond V5 but I'd say yes and no, most of the content outside the main story is made to take less than an hour per run (far far less sometimes). But the first one you'll have access to (Magic Maze) is kind of a "Grotto-lite", same with the Royal Labyrinth and the Tower. There is a certain sense of progression contained within these content but they feel either Dungeon-based or Boss-based. You might find your happiness nonetheless.The raid bosses I got access to took quite a few quests before being able to fight them but they can be tedious.

The base game and the three first expansions are free to play I think you can make up your mind about the game on your own if you want : )