There are multiple ways to "pretty up" the dashboards, and in turn the phone app. In addition, there are ways to just make the stock stuff look better by adjusting column widths/heights, colors, and icons. If you are interested, I can link some of these, or you can search for them on these forums.
What are you trying to back up? Do you have many devices or automations on your hub yet?
I have never used hub linking, so I cannot answer that. But why would you not put your sensors on Hubitat too? That way it is all on the same hub and all local no matter what?
I know lots of people do link hubs, but in my opinion there isn't much need for it. There could be a edge case here or there where you have a cloud device in Smartthings that doesn't integrate well with Hubitat, but those things are few and far between, and the gap is getting smaller all the time.
I would also add, you should give Hubitat more than a days chance. It took you a long while to get your Smartthings where you wanted it, so maybe give Hubitat a week or 10 days and then see what you think?