Oh man you need to do the psijic skill line . Your going to love it.

Hundings is still a good set. Nm got nerffed iirc.
I have not had time to do anything in murkmire except the prison

Hunding's Rage is a good crafted set, but the best sets still come from trials such as Cloudrest or HRC. If the Summerset or Murkmire zone sets seem interesting to you for your build though, you can find them on guild traders too, without having to farm them up yourself or even having to buy the DLC, and the crafted sets can of course be crafted for you.

Dungeon requirements haven't been changed iirc, perhaps you got thrown into some of the DLC ones which are always harder than the normal vets. Rewards are random, and RNG can be a pain in the butt. If it's dungeon loot you're after, queue with a guild group or a bunch of friends who can all trade you gear instead of leaving group without ever saying anything like many randoms .

If you haven't enjoyed Summerset quests, you're not going to enjoy Murkmire either because it's just a smaller zone with less quests that are also less grand. Try to do the main quests on Summerset first without being sidetracked by side quests, maybe that will make the experience a bit more cohesive and interesting. Same goes for Murkmire, there's a cheesy treasure hunting swamp adventure questline, but if you find that boring you can always do the Blackrose Prison arena, or do daily quests for motifs and gear.

The Psijic skill line, while interesting to play around with and has usefull skills, has a super braindead way to unlock it, but if you can sit through hours of riding across random locations in the world, you could find it very interesting and refreshing (the last guild skill line had been a good while ago, and Psijic actually has active abilities ).

It can be easy to get lost after a break, but you can still do the same types of gameplay you enjoyed earlier. Cyrodill is still there, with new structural updates, battlegrounds are part of the base game now, you can always quest and explore new zones and play through their stories, you can run the new added dungeons (probably normal only for now, until you get back into the flow), you can level Jewelcrafting and start catching up with other professions and the new motifs, and do whatever else you're interested in. Welcome back to the game btw, hope you'll find your footing again and figure out which content you're most interested in doing from all the updates since you left

The current setup for dungeons is like this:

You can manually travel to any dungeon at any time, but if you use the groupfinder, Normal Dungeons unlock progressively from level 10 to 45. Veteran Dungeons first unlock at level 50, but the harder Vet DLC dungeons unlock at CP 300 due to their difficulty.

The base game dungeons were overhauled at One Tamriel but have not increased in difficulty since. DLC dungeons are markedly harder than the base game dungeons.

Dungeon bosses now drop a set piece from one of the three sets in that dungeon according to their type, unless that boss also has a unique drop in their drop table.

Mini bosses will drop either a hand, waist, or feet set piece.
Bosses will drop either a chest, shoulder, head or leg set piece.
The final Boss of a dungeon will drop weapons or jewelry.
Normal mode dungeon bosses will drop blue-quality items.
Veteran mode dungeon bosses will drop purple-quality items.