iMessage sync issue. Specifically refuses to sync my biggest group chat

Hi everyone, this is going to be a long one. I'm frustrated and at my wits end with this and I've tried everything I can think of. About a week ago, I bought a new MacBook Pro and it's now running MacOS 15.2 Sequoia. I also use and own an iPhone 14 Pro, running iOS 18.2.1. I started this venture wanting to copy my messages over from my old MacBook Pro to my new one but realized that MacOS 10.15.7 could no longer simply copy paste to the new OS. No big deal. To resolve this, I purchased iCloud+ and enabled sync on my old MacBook Pro, so it would upload and back up all of my Messages to iCloud. It successfully did so, and I turned on iCloud Sync in Messages on my new Mac to download everything from the cloud to my machine. I let it sit for a couple nights, and I noticed two things: 1) it was missing some attachments from some conversations. 2) it was missing whole text messages (not SMS, rather convo messages of text). It also would not sync more than the 3 most recent messages in my biggest and most frequently used group chat, and those three messages were sent after the iCloud Sync was enabled. I tried iCloud Sync on my new machine several times, but never really got anywhere past this. So I figured, maybe my old MacBook did not upload everything. which is fine. It uploaded whatever it had, and my iPhone would supply the rest. I enabled sync on my iPhone and let it sync, and it uploaded and downloaded items. It added about 400 MB of data from the prior sync, so my suspicion was right. Now the message history should be complete! I disabled sync on my new macbook pro and deleted chat.db and all associated files, folders, plists, etc. essentially started fresh. I enabled sync for a second time and let it download. So, it managed to fetch those missing conversation chunks and it seems like all the attachments are there (I can see it within the storage section of settings). HOWEVER, it refuses to download my main and largest group chat. I don't see it anywhere in the message list on my MacBook despite it being the most recently used conversation on my phone. It has not disappeared from my phone so that means it is still in iCloud. I'm really annoyed that iCloud sync does not work properly, especially when I subscribed to iCloud+ in order to fully use it. Any ideas on what I could do here? Any settings or library files I could erase so I can start fresh?