Looks like a great project, looks to be a solid frame that can still be saved! Take your time and put in 110% effort into sanding that rust. It will be well worth it.
One thing I did after treating the rust on the underside of my TJ was use an entire can of PB rust inhibitor. It kind of looks like someone put Jerri Curl on the underside of your jeep but it’s two winters in without needing to be reapplied. And no rust peeking through.