Assuming the primary issue you are having with the Kinesis Advantage is that the thumb cluster is too high vertically and that you want to stick with a commercially available solution as opposed to a custom build, consider purchasing a Maltron. I would have recommended a DataHand Pro II but the manufacturer is sadly long gone. My understanding based only on third-hand accounts and internet "lore" is that the Kinesis design was likely "borrowed" from the Maltron with a few changes made partly to avoid legal problems and mostly to allow for a more automated assembly process and a resultant lower price point. One of the design changes was the very poor ergonomic choice of raising the thumb cluster keys vertically. I use all three keyboards with my preference being the DataHand followed very closely by the Maltron with the Kinesis a distant third option. Between the Maltron and the Kinesis, I strongly prefer the Maltron, mostly due to the thumb cluster placement and partly due to the, in my opinion, significantly superior geometry of the rest of the keys, though the central bank of keys do require rotating the entire keyboard slightly clockwise or counterclockwise depending on which hand is being used to avoid rather uncomfortable left or right bending at the wrist. Typing for significant periods of time on the Kinesis results in, at least for me, some cramping and discomfort related to having to hold the thumbs up physically while typing. No such issues with the Maltron, though I can still type for far longer on a DataHand. Having worked in the past in a high output production setting requiring constant transcription/data entry, I would strongly recommend against procuring any keyboard that is not "split" and/or has a "flat" arrangement of the keys.
Note the relaxed hand position while resting on the home row keys of the Maltron:
Note the tendons obviously under strain while resting on the home row keys of the Kinesis:
The DataHand, just for comparison:
