I've been trying Monarch as well and like you said, it seems to be the most similar product from a user interface/functionality point of view. I've used Mint for close to 10 years and mostly liked it, but I've always had some issues with their category rules getting applied correctly. For the life of me, I could NEVER get Costco to correctly apply. I just found Mint to be buggy at times for whatever reason. Maybe, in hindsight, they didn't dedicate enough engineering resources to it.
I also wasn't able to connect my 529's either at first on Monarch, but tried again and it worked for me the second time. FWIW, I had opened another tab and logged into my account there before trying a second time, but I doubt this is what fixed it. No issue connecting any of my other accounts except one (which gave me a horrible time at Mint, too, so I don't think it's on Monarch's end).
I'll let y'all know what I think about Monarch during my trial over the next several weeks, but I have to say that it's somewhat of a relief to not be bombarded with offers and ads everywhere, so that's an initial positive.