On Romance
While I sometimes read (and enjoy) a MUCH more intense version of Madara than I would write for my own work, I strongly believe he would NOT BE DEGRADING or abusive to his S/O. He's all about love. Accuse me of wearing rose-tinted glasses, but here's my take.
Madara is difficult to write since he's so complex. He isn't some static character who's always optimistic, gloomy, violent, or whatever. He changes with time, growing more cynical and bitter as he lived through his tragic life and came to conclusions about the truths of the world. I will try to consolidate what we know about him and extrapolate that into what romance with Madara might generally be like.
He Respects the S/O
It’s really easy to make him out to be arrogant and demeaning, or controlling. He's allowed to be arrogant when he's stronger than 99.99% of the world. Go re-watch his fight vs the Shinobi Alliance or the 5 Kage if you don’t think he has grounds for arrogance. He is simultaneously the hardest on himself. He's extremely self-deprecating when reminiscing on his past failures and transgressions, both as a child and an adult in interactions with Hashirama. Part of him likely believes he doesn't deserve you or to be loved. Of course he would naturally cherish you when you finally become a part of his life. How could he constantly insult or degrade you?
He is 'controlling' in the sense that he likes knowing where you will be/your schedule. It helps him plan when he can spend time with you. It also better helps him keep you safe. Remember Madara is so strong because he wanted to protect what's important to him, but he didn’t even force his clan to follow him when they abandoned him. Madara wants people to CHOOSE him. He wouldn’t force you into things. He wouldn't beat or coerce you either, especially not when that was what was painfully done to him.
By the way, he'll remember those special dates better than you will.
Is Observant and Sensitive to S/O
During kid Madara’s 2nd meeting with Hashirama, he was able to tell Hashi was feeling down from behind before even talking. And then Mads repeatedly tried to get him to spill what was bothering him in an attempt to console a potential enemy shinobi! Deep down, Madara is a kind sweetheart. He'll be there for you when you need him and there in the background when you don't realize it. He will support you in your endeavors through thick and thin.
He absolutely teases you though, in good faith. Whatever he’s mocking you about is one of your quirks that endears you to him, not something he actually judges you negatively for. The same cheekiness from his younger days is still alive in him somewhere. Don’t worry, he won't roast you as hard as he can where he knows it hurts. Hmm unless you're in a bad fight and he's feeling really petty, then I can maybe see him doing that. His words could cut more than his fists.
Is Almost a Different Person with His Loved Ones
This is another reason he’s hard to write. While there’s no doubt he can be terrifying (which was how almost everyone in the world saw him), he can be kind of silly with those he’s closer to. He is indifferent or aloof with strangers. He was likely at least civil with others he had to deal with in public and condescending and/or hostile with those who gave him a reason to be. This man’s presence is felt wherever he goes, so of course the average person would have reason to find him intimidating.
In fact, you might occasionally find Madara intimidating too when he's in one of his moods or contemplative. It's how he is, even when not trying to be hostile or domineering. He's still softer with his loved ones than anyone outside his small inner circle. It would break his heart if you were truly afraid of him.
On the other hand, he may be awkward at times. His inability to effectively express his strong emotions to you frustrates him. He wishes he had a better grasp at social interactions, in particular when it comes to you. This guy is just too intense and doesn't know how to chill.
Cherishes the S/O
He doesn't have much preference when it comes to appearances (body type, skin, hair colour, etc). He wants someone who's easy enough on the eyes? But beyond that, looks are superficial to him. Some people think he is vain because of how he mocked Hashi and his bowl cut/fashion sense. But really? We all know Mads held Hashirama in higher regards than he did anyone else. Madara connects through more intangible traits like values, the mind, strength, and such. He’d enjoy whatever fell out of your clothes if he loves you and doesn’t understand stuff like body insecurities. His "type" would be someone who is family-oriented, placing their children and kin above other priorities, someone who likewise supports him and wants to build a life with him. And someone who can challenge him a little.
Madara is loyal. He values his bonds and was honest until the end. Once you're his, he's also yours. You'd be super precious to him, the light of his life kind of thing. Your presence in his life inspires him to be better than he is.