Confused by lengthening wake windows on 3 naps

My son is turning 6 months this week. He's been doing great on 3 naps for a while, and finally able to lengthen one of his naps reliably (typically the 2nd one). Designated wake time is 7am, and bedtime is 8pm. Approximately 2.75 hours of daytime sleep.

Schedule has been 2.25/2.5/2.5/3 for quite a while.

He's starting to lengthen his wake windows/fight naps if put down at his normal times, but this has left me confused. How do I manage longer wake windows in terms of sleep needs?

Do I shorten his naps even more? Do I make bedtime later or his designated wake time earlier?

I don't think he's ready to drop to 2 naps, especially since he isn't quite 6 months yet, but we're starting to have EMW again so I know he's not getting enough wake time.