Kuuga was similar to N-Daguva-Zeba? When the fuck was that?

N-Daguva-Zeba was supposed to be a evil version of Kuuga(or was Kuuga a light version of him?). Their powers come from the same source and they look pretty similar. Though that could just be the usually "All Kamen Riders share a connection to their villians" thing, I thought it went a bit deeper in Kuuga's case.

That didn't really feel any deeper than OOO using medals like the Greeed or whatever was going on with Gaim.
The similar powers theme is something that has always been present in KR, so Kuuga and Daguva's connection felt shallow, sharing no real similarities. Daguva wasn't a dark reflection of Kuuga. They were just characters with similar power sources, par for the course with KR.

Had they established a grounded connection, things would've been different.

Maybe Kuuga's lack of another rider made me go crazy and I started fabricating a rider when there actually wasn't one?

No, the fact that Kuuga and Daguva were the same was well established by the show imo. Matrix may not have gotten that impression (which probaby is a contributing factor towards him not liking the show) but I think they did a good enough job drilling the similarities into the viewer.

No, I got that impression.
They just did a shit job at establishing it and I disregarded it.
You can't just say "THEY ARE TOTES SIMULATION YOU GUS" and proceed to act like it's law while doing absolutely nothing with it.

N-Daguva-Zeba was kind of the epitome of show, don't tell. They spent the entire final 4th of the show trying to hammer in everything in a connect the dots style way, but rather than do anything that makes sense, they sat on their hands, showed off Daguva's name in Grongi being similar to Kuuga's, and then shrugged and expected that to be enough.
Daguva did not have any character, and if you say he did you're a fucking liar. There was no real connection beyond their powers, which is the weakest connection possible since most Riders already have that kind of connection with their villains anyway.
With the lack of a proper character to Daguva, they were only able to establish tenuous and strained connections between them, so I never felt like he and Kuuga really shared anything.

Yea yes but use it as story sibling were separated at young age and younger sibling shows up as secondary rider with antagonist or is being controlled by big bad then yea it can work

I dunno about that. That kind of story is incredibly typical. It wouldn't really be that interesting, honestly.