Is there a way to upload my ipad backup to the cloud if it was saved to a PC first?

No, you cannot move it to iCloud to use it to restore the new iPad if that is what you mean. You can certainly just move it to iCloud for storage, but you won't be able to do anything with it once its there. There is no way to incorporate it to the iCloud backups.

You can buy a USB-C to USB-A adapter to connect the new iPad to the PC and restore the backup to it.

link-> USB-C to USB Adapter - Apple

No, but you don't need to. You can simply backup to iCloud directly on the iPad by going into Settings->Account(top of settings)->iCloud->iCloud Backup->Backup Now.