Listen, mate, you don't need debate advice or communo-evangelical advice right now. You need relationship advice. There are contexts like working for capitalists or trying to organise radicalising liberals where you need to be shrewd about what you say and how you say it, but a relationship, especially a long-term romantic one, is not the place for being tactical or tactful. You need to be able to be who you are, and speak your whole mind, around the one person you expect to spend your life with.

With that in mind, you can see that your problem isn't Stalin or spirituality, it's how you approach conversations with each other. You said it yourself: You can smell the whiff of condescension from each other when you come to a disagreement, which means either that your do not respect the viewpoint being endorsed, or the core philosophy and worldview behind it. This is a serious point of tension, and needs to be treated as such. As a breakdown in communication due to an inability or unwillingness to move past a difference not of facts, but of outlook.

The difficult conversation you have to have isn't one about how the Holodomor is propaganda, or you promising not to praise the USSR. You need to sit down with your partner and say, "I know we don't see eye to eye on this, but this is important to me, and it's clearly important to you as well." You are both politically-conscious individuals and the politics you have arrived at are an expression of how you engage with and understand the world. You identities are bound up in, and so you need to navigate the topics with care and in a way that a disagreement about the topic doesn't become a negation of your sense of self and feel like a rejection of your whole belief system.

Basically, this is not an online argument, so please don't approach it like one. It's not a problem that can be solved with a magic answer. You have to essentially be willing to say "I will disagree with you without disrespecting you and I ask the same of you in turn. We both love each other and have the best of intentions and want to understand how best to better the world, and I want to share my perspective with you and help you understand why I see the world the way I do."

Be willing to avoid certain topics if your partner isn't comfortable with them currently, and set your own boundaries as well. Have a negotiation about what engaging on this topic looks like, and what you would like to do together to help understand each other better. i.e. try to prepare a neutral ground based on trusting each other enough to have difficult conversations.

The bad news is that if you can't come to an agreement about how to have the conversations, then that is an indication of a fundamental difference that you will not be able to resolve, and you will have to ask yourself how willing you are to live with that. But try. If you have that level of trust, it will hopefully work out to step 1 of the process.

I am not trying to be an ass, but I'd like to ask for some context in the form of a harshly-phrased question before I answer you. Namely, how did it get to the point of y'all planning your WEDDING before topics like this were seriously discussed?