I could have written this post! My DS who is almost 12 weeks have gone through the exact same thing you are describing. I also have fast letdowns. Even though your LO is managing the flow he must be gulping a lot of air. You would be able to hear him gulp it. When you burp him some milk comes back up with the trapped air. When my fast letdowns got fixed LO was burping normally with minimal spit ups. Sadly, fast letdown is back for me & so is his gas & spit ups.
My LO also spit up clear or clumpy cottage cheese looking normally 2-3 hours after the feed & this has gone down considerably in last 2-3 weeks.
My suggestion is to burp him 3-4 mins into nursing when the fast letdown has passed and again when he is done at the breast. My DS required about 5-10 burps. And I wait for about 5-10 mins before putting him back on another side.
I also held DS upright for 30 mins after feed which means no diaper change also. If you are going to change his diaper after 30 mins try not to push his legs into his tummy, I sway his legs side by side & change him. And just like your LO mine slept fine in the night & I think it is since the night sleep is different from day sleep. Also they normally fall asleep or suck it gently during the night feed & this helps them gulp less air & hence minimal or no spit ups.
Because he constantly needed burps or will spit up, he too was not napping in the crib at all. He would wake up 5-10 mins after putting him in the crib. So I held him in my arms most of his naps. Every once a while I would go out of his room with him in arms to eat. He was used to his dark room & sound machine & would wake up sometimes outside his room. If your LO is ok with light/noise then I would suggest to wear him in sling/baby carrier.
I also started nursing him in his dark room where he would be drowsy & will suck gently. I noticed that when he was sleeping while nursing during the day he didn't spit up though he needs burps.
Things do improve with time though. His day time sleep improved with age & he started sleeping in his crib. Of course, I still have to go pick him up to burp him in the middle of his naps multiple times, but he naps in his crib if he doesn't need more burps. I still hold him for part of his nap/s as my fast letdown is back again
I also found that eliminating dairy from my diet helped with his spit up.
Lastly, I visited his Ped to rule out GERD. My LO was never fussy at the breast or arched his back or acted fussy during the day except in the evening which is fussy time for many babies. But I wanted to make sure it is not GERD.
Hope this helps! Good luck!