One of the key differences to other instances is that Ukrainian culture was not really tolerated in the USSR, and definitely not in the imperialistic cute of Russia today. Rather, Russia learned from the demise of the USSR that it's better to remove cultural and societal differences within its nation. And it's going the same route as other empires have in the past and are doing today. Enforcing the use of its language, forcibly educating the youth in Russian culture and schools, often by literally abducting them from their Ukrainian homes and relocating them to Russia into Russian families. Kidnapping a people's children is literally one of the five acts that make a genocide, according to the UN genocide convention from 1948.

There are many examples to show that putting peace above all else is dangerous. Using your example, the Welsh were violently suppressed and exploited for centuries by the English, following a similar scheme. The Welsh language was forbidden to teach, Welsh traditions were replaced by English, and power of autonomy was transferred to the English. Nowadays less than 20% of the Welsh can speak their language, and that is after the "resurgence" and it being allowed to be taught again.

Other examples are Native Americans, the Armenian genocide, Czech Sudetenland, Poland, oh, and remember what China is doing in Xinjiang? All of them posed no or little violent resistance in exchange for promises of peace and cultural autonomy. It rarely takes more than a few months before contracts with powerless people are broken.

Nations are just figments of imagination, but are an expression of communal will of a number of people. Modern ( as in last two centuries) concepts of human rights revolve around the freedom of people to choose how to live their lives. We usually consider it admirable if people are allowed to freely live their chosen cultures and tradition. We also consider it proper that people are allowed to choose what kind of society they want to live in/migrate to. We also grant asylum to those forcibly prevented from living their way of life and being persecuted in their home nation.

Nations are a construct allowing specific sets of societal rules to be applied to a large populace. An internationally recognized nation is also considered integrally protected, even if one nation might not agree with the internal societal rules of another country. These global base rules are very bare bones, but they are one of the big reasons we all get to live in the era dubbed "long peace". Yes, there's still wars and ugly conflicts, but at least there has been no major total war involving superpowers. Even the Ukraine war is luckily still limited in scope. Were Russia to unleash their full military might, it quickly would devolve into a humanitarian disaster not seen since the world war.

Why does the arbitrary nature of countries favor the invader though? What about the argument that Russia is an arbitrary country that doesn't need to violently expand into neighbouring countries?