Note that D:OS is not actually isometric.

Underrail (https://www.gog.com/game/underrail) is from 2015. Although it looks worse than good old Fallout. Still a very good game IMO.

Wasteland 2? Was recently free, some codes might still float around. (also not "isometric") ;-)

Most isometric RPGs are not really open world. None of them are as open as Skyrim, due to the heavy level scaling Skyrim uses. Any RPGs I list use no (like D:OS) or very litte level scaling (only in random encounters)

Since you disliked D:OS 1, i will not recomment D:OS2, It has many similar problems. In a sense it is even more closed, since each act is set in one big map, where your level (and equipment) decides where you can go. I especially dislike the equipment scaling, requiring you to scour the merchants after each levelup. Also it is lacking the "whimsy" of D:OS1.
But the battle system is really unique and a very strong point for the game.

Pillars of Eternity Deadfire. From mid-game on you can actually start to expore properly as not get you ass kicked everywhere ;) You can see the area level before entering, so not much trial-and-error is required.
I cannot say much about PoE 1, I could not get into it.
Pathfinder Kingmaker is more closed. Several regions on the Worldmap unlock bit by bit. Also the timers (some are hidden) can be annoying. Still a gread game.

I would also include Baldur's Gate. Especially BG1 is very open once you are done with Nashkell, with lots of areas to explore. I would say at least 50% of overworld areas (and even some dungeons) are "fluff" and not required to beat the game. Some don't even have quests, but still nice stuff to find.
The levelcap also means that you do not have to hunt for each and every subquest. But the game can be very tough to get into, especially while you are low level.
BG2 sadly lost much of the open-ness and most of the fluff areas, but is still quite non-linear.

Neverwinter Nights might also be worth a look. Quite linear, but with some choice in area order.

All of these use a real time with pause battlesystem which is *very* different of D:OS

There are many others, Tyranny, Wasteland 2, Icewind Dale, very old-school are the Spiderweb games, even the modern ones.

I'll but in another vote for Underrail. Very open-world, and strongly rewards exploration; also has a strong central story and excellent game mechanics that allow for many different approaches. You could also take a look at the recent Avernum re-makes (Escape from the Pit, Crystal Souls, and Ruined World); they're quite a bit cruder since the re-makes still hew closely to their origins, but the overall structure is quite close to the open-world games you mentioned in that the player is basically given some general leads to start with then turned loose on a massive world to explore.

Also relevant.
It's one of those cases of "close enough", since most terms describing similar perspectives haven't caught on in the vernacular. If it's not first-person, top-down or side view, it usually gets lumped into the "isometric" category, even if the camera's pitch can be freely adjusted. Still, I often enclose the term in quotation marks, just to avoid pedantry (no offense).

EDIT: Apologies, I had to delete the link tags from the quoted comment, as it was causing all of my post to display within the quote for some reason (with both hyperlinks messed up, to boot).

Not modern, but fulfilling all other requirements would be Ultima 7

It's isometric, it's open world and it is very, very good. Plus it's discounted at the moment. If you haven't played it yet, do. (and if you like it, you might proceed to the even older Ultima 6)

The original Divine Divinity was pretty open, other than the very beginning and the very end. Sacred seems pretty similar, although I haven't played much of it.

sorry, I can't think of anything this decade offhand.

Thanks for the thoughtful response. It's disappointing more developers haven't been as ambitious in this regard with openness.

I have accrued a backlog of some of the remastered classics you mentioned so maybe Baldur's Gate will be next or Wasteland 2.

I can quite concur with that. One of the great games. Has a lot of lore, so if you like making notes, drawing maps, finding secrets it’s good too. Combat isn’t that great which is the only downside. I liked the container init, but I know some did not get on with it.

Some that haven't been mentioned that you may or may not like:

Sega Genesis' Rings of Power
Might and Magic
https://www.gog.com/game/eschalon_book_i (episode 1 is free on gog) (I didn't like it a lot, but it was decent fun)
Many of the spiderweb games have lots of branches if not totally open world
I enjoyed the Risen series. I never finished one have, but what I did finish was very enjoyable. I always break my character by making him too chatty and not enough killy.

Maybe Hexplore? It just came recently to GoG.

Pathfinder Kingmaker, Grim Dawn, Pillars of Eternity, Pillars of Eternity 2, Wasteland 2, UnderRail, ATOM RPG, Diablo, Planescape Torment, Baldur's Gate, Baldur's Gate 2, Icewind Dale, Icewind Dale 2, Divine Divinity, Beyond Divinity, Fallout, Fallout 2, Arcanum, Avernum, Avadon...

If those don't get you started, none will.
EDIT: Modern ones are at the beginning.

Hey, thanks for pointing that out. Fun game.

-Arcanum is the best!

-ATOM RPG, is quite an open-world rpg game. You can even buy/find more then one type of cars. ;) And you play the game, right from the start, pretty much the way you want. Go almost anywhere you want. The limits are your skills and fighting capabilities.

-Stygian: Reign of the Old Ones, not really open world game. But one heck of an rpg game.

-Kingdom Come: Deliverance, not isometric for a second. Open-world-like-Skyrim game. Absolutely superb game!

The ones that got me into the RPG genre were Fallout 1 and 2. After that I played Arcanum: Of Steamworks and Magick Obscura.
Arcanum I can't be sure, but F1 and 2 are open world, though some places could be (seriously) above your level should you get there early on.
They're not from this decade, but especially F1 and 2 are timeless classics.