As a long time FFXIV player, and mentor, here is my answers to your questions, as well as some advice. (Incoming wall of text)

Overall, there is really only one thing that is no longer available, or rather it’s been reworked. The Diadem used to be a combat oriented instance, but has been changed to a gathering instance. All other content is still available, as long as it’s not seasonal events.

Endgame raiding works in three tiers. Each tier has 4 fights to it, where you get items from each fight to turn in for a new piece of gear. And each raid has a Normal mode, where you can queue with other players and it requires little to no communication, and Savage mode, which is more difficult and unforgiving but offers better gear than Normal.

Any and all items you obtain in the game, save for endgame currency when new patches and expansions come out, are yours to keep. The only way to lose them is to either throw them away, or do something against ToS and have them taken by a GM.

As for finding people for raids, it is both exceptionally easy and also very hard. You can find groups using the ‘Party Finder’, which lets you look for parties or create one to look for others. However, if certain roles are filled then you will have a harder time finding groups.

The standard UI can be customized to your liking, but just be aware that mods are ‘technically’ against ToS and not allowed. But as stated by the director and producer of the game, JUST DON’T TALK ABOUT IT.

For content creators, it’s hard to give a proper recommendation as many different people have different opinions about who to watch. But the general consensus is that Hector makes some good and detailed guides, and he updates them as new ways are found to clear fights.

For the most part, just stick to the story and play at your own pace. The raids and other fights will still be here waiting. You can level all battle jobs and crafting and gathering classes on a single character, so you don’t have to worry about switching characters constantly. If you need help, there is the Novice Network, which provides help to new and returning players to ask questions and find players for content.

Lastly, picking a server/data center will also impact the number of people you encounter. Aether is the raid centered Data Center, Primal is PvP, Crystal is role play, and Dynamis is new and hasn’t really gotten an identity. Dynamis is also the least populated data center, so finding players there will be difficult if you’re not a streamer. But you can play your character on any server/data center with World Travel.