Like I said, arguably. Show me some data that says that the opposition has grown above 25% (arbitrary, you may understand what I mean)

In 2020, the support for Alexi Navalni was around 20%, which, as the article notes, was despite the fact that all of the available Russian media--which is entirely controlled by the state--demonized him as a traitor to Russia. If you had a reasonably free media in Russia that was free to report on what Putin was really doing, then it seems likely that support would have been higher.

But if opposition leaders keep getting jailed, or commit suicide, every time their public support breaks out of the single digits, then yeah, duh, of course you aren't going to see opposition about 25%.

There are diplomatic avenues to solve this problem, so maybe Ukraine can solve the whole thing, in the interest of preventing future wars.

You're missing the point entirely. Sure, it's not rape if you consent. And you can stop a rape that's already happening by saying, okay, sure, I consent to this. But, really...? That's the direction you want to go here? Russia could also stop this at any time just by pulling troops out, and giving Ukraine it's own land and kidnapped people back. Russia can prevent future wars by, I dunno not invading other countries. Why should it be the responsibility of the victim to negotiate with the aggressor?

Again, for the sake of argument, assuming that Russia itself was victimized during the fall of the USSR, and assuming that Putin is seeking to redress that, rather than him trying to take over the whole old-bloc, then is there any other peaceful path?

Russia was victimized by Russians. Not by the west, not by Ukraine. These are all self-inflicted wounds, not some grand conspiracy by The West. The former Soviet states didn't want to be Soviet states, because the Soviet government had always been complete dogshit. When the USSR broke up, the politically powerful and connected systematically looted the country of wealth; it wasn't western governments and companies that looted the country. So if Putin wants to fix that, he needs to fix his shit, not blame everyone else for the problems that Russia created for itself. But that's not what he's been doing; he's trying to mask internal problems by claiming that it's an external enemy.