Everything has been going well overall with the schedule. I'm having no problem with going to sleep at night and waking up 3 hours later, feeling refreshed and alert. I've even moved my core sleep back a few nights when I wanted to stay up late for doing something. I've gone to sleep as late as 2 and woke up at 5, and continued the schedule as usual, taking my 90-minute nap around lunchtime. I've actually been able to move my nap some as well on days when I had nothing to do in the morning or wanted to be available at noon for something. I originally wanted it to be about 11:30-13:00, but one day I did it about 9:30-11:00, and another day 10:30-12:00. No problems from this, so it seems that I'm getting relatively adapted and having some flexibility in my schedule.
Although I'm having no trouble waking up at night from the core sleep, I am still sometimes getting sleepy about an hour after waking up from the core. Like my body still wants to go back to bed at the early morning time when I'm typically sleepiest. So I have not yet overcome that urge to sleep then completely, although I am getting much better at waking up with the alarm.
I thought I might try to abate the sleepiness some by taking a 30 minute nap when I'm really tired. I've found that adding in short extra naps doesn't really do much for me, though. I wake up feeling just as tired as I did when I went to sleep when I nape in the early morning; evening naps are better and usually refreshing. I think that, again, this is just due to me being most sleepy in the morning. Taking an extra nap then doesn't really seem to do anything, and doing so probably just prolongs the difficulty of getting over sleeping then. The best thing to do is to just keep my waking time as consistent as possible and do something to wake myself up then, to make it clear to my body that this is no longer the time for sleep.
I know other people have said they had success with extra naps, but I'm now thinking that it might be a futile addition that doesn't really solve the problem. Perhaps it depends on the schedule. Those without a core sleep who are training themselves to always sleep for 20-30 minutes might do better with it.
I feel that I've made some progress in no longer being so groggy immediately upon waking from the core sleep (something that was a real problem for me with previous attempts), and I've at least trained myself to do something there. Or maybe this schedule just works out better for me. I feel like it hasn't been so difficult at all compared to trying to do other things in the past. There are many reasons that could be, and most of it probably has to do with having previous experience and having developed some skills in efficient sleeping and the desire to keep going with this.