UPDATE TO MY TIMELINE
9/23/23 - Shipped through USPS
9/25/23 - Packet received at Phoenix lockbox
9/27/23 - I-751 added to my USCIS account, stating case was received. No pdf uploaded
10/2/23 - Received text with receipt number, stating I-797 letter will come in the mail
10/3/23 - Check cashed
10/4/23 - Status updated to "Case is being actively reviewed by USCIS", had notices uploaded on my account with the 48 month extension + a notice that my previous biometrics are being reused
At the bottom of my extension letter it says my case ended up at the Texas Service center... and after checking the USCIS' processing times for TSC, I realized this might take a lot longer than I hoped for 🤣