No, I played them all on 360 too, but I never got any of the DLC, so I'd love to buy the whole series DRM free with the DLC. It's a dream.

That's not exactly the case. I don't think there were a huge number of copies made, it just bombed. The smallish number of copies made still outstripped demand at the time so it wasn't hard to get hold of. It's become a cult classic over the 25-ish years since it released, so it's now a rare game because demand outstrips supply.

Some of the DLC(ME 2/3 especially with regards to the storyline stuff) is pretty good, so I hope you get to experience it someday.

Civilization II was better than the original. Apparently another sequel, Call To Power is the best of all these (I haven't played it yet).

Is it "not worth playing" bad or just "not a patch on the original" bad?
I think Civ 2 was better than 3. The worst part of the game is the mess that rail makes across the maps; ignoring this, though I think the gameplay is superior.
I could very well be suffering from rose-tint, though, as I haven't played it for decades. (I did play it to death when I first bought it, though. :)
My rig is not powerful enough to play Civ 4 for very long, but I kinda prefer 3 to it. (Though the stacks of doom suck, and the scenarios of 4 are a great addition.)
I think the Invisible War demo that was released contemporarily is much more fun than the actual game. (Obviously the narrative is puddle-deep.)

A combination of both. I don't own it although I did play-test a few hours on a friend's computer I was fixing up last summer, and I found it was a huge mess of unfixed bugs that still remained +2.5 years post launch, eg, players got stuck in "crisis mode" which means you can't save and had to reload, there are no spoiler tags for GOG forums but one big one was if you went to Old Slave's Block in Bloom and found Artaglio's standard before getting a pass from the mercenaries in Chiurgeon Slump they will never let you in. If you saved after that, you're screwed and have to start a new game. Many other minor but still irritating issues were widely commented on in the forums and from what I gathered after a certain time elapsed, due to poor sales the devs just gave up and walked off without properly patching the bugs up. Unity Engine problems include random performance drops plus the text was strangely rendered at 4K for everyone and then downscaled and ends up looking blurry at 1080p on a 1080p monitor.

As for "compared to PT", I found the game was more boring and the devs completely overdid the "wall of text" thing thinking that was what people loved about PT when it was more about the completely unusual plot, settings, etc. The combat is now turn-based and felt far slower (like padding than clever tactics) vs the RTwP original. I'm sure it has its fans, but it really didn't "click" with me anywhere near what the old Infinity Engine games did.

Edit: I just checked some recent reviews that were made 3 years after launch and they're still complaining about the bugs : "TL;DR It's a big let down, bugs kill the game. So... where do I start... I think that the biggest issue is that even after 3 YEARS after the game got released it still have game breaking bugs. How is that even possible? I've had multiple issues where: character would dissapear, quest would not finish even though it's done, dialogue options would disappear, items would disappear or a quest would not progress even though I did everything as in walkthrough." and "Sad that this quest is STILL bugged in 2019. I just started a game to play this and ran into the bug where combat breaks and I can't attack at all". So yeah at this point it looks like the devs have abandoned it for good with no intention of properly patching it up.

I never played the demo, but i'll check it out if I have time...thanks for the heads up. :)

That's a poor result, especially if the developers want to crowd-source a new title.
I suppose the fan community is unable to compensate. :(