Cat E ALO here, currently waiting on O-4 board results... have gotten the same advice above regarding PME complete, 70+ annual points, no bad years, and “leadership positions” to get a DP to O-5. Not that hard to do, and selection rate with a DP is great.
The issue I have seen for guys not getting to retirement as an O-4 has been when they have breaks in service. I’ve seen it more than once where they were twice passed over for O-5 but didn’t quite make it to 18 years for sanctuary. I almost had this issue coming from AD... took 9 months to gain me into Cat E, had a 40 day break in service, and ended up successfully doing a BCMR to get it fixed.
It’s also not impossible to make O-6 as an ALO, but the odds are pretty slim. You probably have to be in a “national level” leadership role, SDE complete, and a few of the current O-6s likely need to retire first as well.