sleep/nap schedule help!!

working on developing a schedule for my 4.5 month old. he wakes up everyday at 8 am, and goes down for his first nap by 10 or 10:30 at the latest. my problem is the lengths of his naps are soooo inconsistent. on a bad day they're only 30 minutes, and on a good day I'll get them to an hour and once in a blue moon up to 90 minutes. this makes it really hard to schedule the rest of his day! i typically have him awake for 1.5-2 hours between naps. should I set times for naps (like 10 am, 2 pm, etc.) or go by how long he's been awake?

he does well with falling asleep awake but drowsy on his own during the day, but his last nap/bedtime he needs every crutch possible. he seems hyperactive and fussy which I know can be a sign of being over tired but after moving his bedtime up to 8 pm with last nap around 6 I'm still struggling to get him down easy at 6 pm.

he will typically stay in his bassinet until he wakes up or I am ready for bed, and then he sleeps with us through the night. so easily 9-11 hours of sleep at night.

I need any advice! is he sleeping enough, should I space out naps more, move up his bedtime (again)???