I have a TL sequence I shot several years ago before I learned about ramping that was shot in AV mode.  It is a gorgeous sequence but has severe flicker and despite using deflickering, I cannot get rid of the flicker.  What would be the best approach to do this?

Hard to say, without seeing the footage.
But in general: Visual Deflicker, an appropriate reference area, not too much editing in Lightroom and - if necessary - some refine passes of the visual deflicker. Normally this gets a sequence smooth.