I don't know what time the appointments open, sorry.* But last month (6/25) I took DD in to get her passport renewed (expiring this November when we are supposed to go to Costa Rica - who knows if that will actually happen at this point) and decided to go ahead and pay for expedited service. The USPS had a sign posted that said passports could take 18 weeks or more with regular service. That was cutting it too close for my comfort especially since I have to decide whether or not to keep the United award tickets 30 days prior to departure if I want to get a full refund. We ended up getting her passport back in
3 weeks! In the interim I saw several news articles saying there was such a backlog with getting passports that some people couldn't go on their trips which had me worried, but DD's passport came very quickly. Was it due to expedited service? I have to assume it was.
But you're looking at April which is a lot farther out than November...
Oh, another issue I had was with the first USPS appointment, DD and I showed up and the worker insisted that both parents (with government issued documents) had to show up for the appointment to go through with processing the application for a 16 year old (it did not say anywhere in the paperwork I looked through that that was necessary). So I had to make a 2nd appointment when DH could leave work to meet us at USPS to try again...and of course that time the other USPS employee never even asked to see DH at all.

Be prepared for both parents to show up, otherwise you need a notarized letter or something like that giving permission for the one parent to go through with the application.
Excellent point from
@CyndiLouWho to bring your own passport photos. My Costco stopped doing them; I ended up going to
AAA for DD's.
*Have you tried changing the time & date setting on your computer to push you to another time zone? I had to do that back when I was searching for covid vaccine availability and it worked great since
Walmart's website populated appointments at midnight EST.