On ergonomics and RSI, tough to say whether some of these things make a difference or not. There's no data. The single biggest difference to me is having multiple thumb keys because that allows you to almost completely relieve the pinkies. The only sideways movement I use them for now is a "sticky keys" style one-shot modifier for Control on the keys adjacent to the home-row. Although having a split board (and standing desk) has been super for overall posture.
[1] https://www.reddit.com/r/ErgoMechKeyboards/comments/infzme/t... [2] http://www.keyboard-layout-editor.com/#/gists/8105b7f97f89fa...