My thesis mostly hinges on the Soviet Union not being democratic, see https://en.wikipedia.org/wiki/Elections_in_the_Soviet_Union.

Exactly, your material and historical analysis of the Soviet Union is based off of NATOpedia.

In short: Wikipedia is primarily edited by white young males of english-speaking countries, so it features the bias or young males of English a speaking countries. This is well-known and even has Wikipedia articles dedicated to it. In particular, source selection on English Wikipedia is mostly taken from western news sources, so it reflects the bias of western news sources. Western news sources present systematic pro-western bias in geopolitically sensitive issues, you may recall the behaviour of western news sources regarding Palestine up to a few years ago, with no media talking of genocide and presenting the occupation of Palestine as a "both sides issue". Other, possibly more egregious cases you may or not remember are Nayirah's Testimony or the media flip on coverage of Russia in European countries, where up to 2022 Putin was said to be a "great governor and Russian patriot" (e.g. Francisco Marhuenda), which is now unthinkable. If you're interested in this issue with Wikipedia, I wrote this detailed post about it some time ago.

I hope you, a self-declared socialist from what I've seen on your post history, will reflect on using mainstream western sources to analyze topics that are sensitive to western geopolitics as is the case for communism, we're well aware of what the red scare in the US entailed and the lies that have been spread about socialism in general (not just the Soviet project) by the US state propaganda apparatus over the past century.

I encourage you to do some reading of my sources, especially Albert Szymanski's "Human Rights in the Soviet Union", which dispells a ton of western-manufactured myths about the USSR using mostly western academic sources.

And why does Russia's life expectancy spike upwards right around the time the Soviet Union collapsed?

On the graph you can see Russia's life expectancy peaked in 1990, then fell for one and a half decades coinciding with the dismantling of the USSR, and then in 2005 it starts to rise back, but doesnt reach pre-1990 levels until about 2015, so life expectancy didnt recover from capitalism until 25 years of technological advances passed. Regarding Brazil, yes, Brazil surpassed life expectancy in Russia during the crisis of the dismantling, I do think this supports my thesis that the dismantling murdered millions (by Paul Cockshott's calculations, about 5-10 million in Russia alone).

Still leaves the question as to why the Soviet Union just collapsed?

If there were a class of owners strongly gripping to power in order to keep exploiting the majority of workers, you would expect very violent revolutions being needed to dismantle the system and remove them from power, but the transition to capitalism in the Eastern Block was overwhelmingly peaceful, which again supports my thesis that there wasn't an owning class enjoying the fruits of others' labour. As to why the USSR was dismantled this is a long topic, and if you're interested in some materialist historical analysis, I recommend "Socialism Betrayed: Behind the Collapse of the Soviet Union" by Robert Keeran and Thomas Kenny. It gives a good historical outlook on how it's possible that the USSR survived something as impossibly difficult as WW2 and the murder of 25 million Soviet citizens (13% of the population) by Nazis, but it was dismantled in half a decade since the start of the perestroika in 1985.

Please, you're patently showing that the reading you've done of the topic of the USSR is superficial and based off primarily western anticommunist sources. I encourage you to keep an open mind and read more about the project that uplifted 150 million peasants in the Russian Empire from extreme poverty to being the second most powerful nation on Earth, guaranteeing healthcare, education, housing, work, not performing unequal exchange or economic imperialism with any sort of colony unlike US and Europe with Africa and Latin America, and helping emancipatory movements such as that of Vietnam or Cuba.

Stop looking for excuses with on-the-spot reading of graphs or moving the goalposts (first wealth and de-jure ownership, then income not mattering, then radio silence about widespread access to social services and essential goods).