You have >2600 attachment files. The top 10 largest (including a couple 85 MB PDFs) add up to 560 MB. That leaves 4400 MB for 2,600 files, or an average of 1.7 MB a file, which is a completely normal size for PDFs, webpage snapshots, and some other documents.
You can search for PDFs within your Zotero data directory from your OS file manager and sort the list by size to see the largest files, and you can paste the 8-character folder name into the Zotero search bar in All Fields & Tags mode to see the attachment items in question.
It will be able to sort by file size within Zotero in a future version.
I also have another paper I'm supposed to be working on but can't, because there's nowhere to put the references.
You understand that you can save as much to Zotero locally as you want, without any limits? File syncing is optional. If you're willing to get by without accessing your files on other devices and are making regular backups of your Zotero data, you can just disable file syncing in the settings.
No, selectively syncing attachments isn't possible, nor are the other scenarios you suggest (which would for various reasons also just be bad ideas).
If you can't afford a Zotero file storage plan sufficient for the size of your library, you can look at finding a webDAV option or using linked files (typically with the help of a plugin like ZotMoov.)
(Beyond that, please post when calm. I understand you're stressed, but the accusatory tone of your post is not appropriate. No one here is forcing you to use Zotero.)
Upgrade Storage
You have >2600 attachment files. The top 10 largest (including a couple 85 MB PDFs) add up to 560 MB. That leaves 4400 MB for 2,600 files, or an average of 1.7 MB a file, which is a completely normal size for PDFs, webpage snapshots, and some other documents.
You can search for PDFs within your Zotero data directory from your OS file manager and sort the list by size to see the largest files, and you can paste the 8-character folder name into the Zotero search bar in All Fields & Tags mode to see the attachment items in question.
It will be able to sort by file size within Zotero in a future version. You understand that you can save as much to Zotero locally as you want, without any limits? File syncing is optional. If you're willing to get by without accessing your files on other devices and are making regular backups of your Zotero data, you can just disable file syncing in the settings.
If you can't afford a Zotero file storage plan sufficient for the size of your library, you can look at finding a webDAV option or using linked files (typically with the help of a plugin like ZotMoov.)
(Beyond that, please post when calm. I understand you're stressed, but the accusatory tone of your post is not appropriate. No one here is forcing you to use Zotero.)