If with one die you bring in a checker and with the second die bear off, you will still be even with 14 checkers. With 554 you are rather unlikely to ever miss. If you play 9/6, the gap will never fill and you will end up wasting a ton from playing 6/2 with 4s or 6/1 with 5s
(1) Your crossover with 9/6 doesn't save a roll. After 9/6 you need 15 crossovers to bear off (8 rolls). After 6/5/ 6/4 or 6/4 9/8 you need 16 crossovers (8 rolls). Same number of rolls in either case.
(2) After 9/6, you have a gap on the 4-point and four checkers above the gap. You can never fill the gap because any one or two will be used to bear off a checker instead. So you miss with all 4s for the rest of the game.
I understand the 6/4 part of the move. Is there any benefit to keeping a checker on the 6 point and moving 9/8 with the 1 on this roll? It probably is one of those too close to matter situations, but vacating the 6 wastes a pip or two on future sixes, and moving your straggler one point closer can’t be too bad, can it?
I just ask because that probably would have been my move OTB and I’m curious how big an error it is.
6/4 9/8 would have been my move over the board as well. XG says 6/4 6/5 is a tiny bit better, and I think the difference is that a 55 next turn will take off 3 checkers if you've played 6/5 but only two checkers if you played 9/8, thus saving a whole roll.