You shouldn't need to open port 8333 unless you want that device to be a full node, so I'd close that port again and increase your router's security level to prevent any network intrusions. Are you running the latest version of Bitcoin Core? If I remember right, some of the old versions of Bitcoin Core can't sync with the network anymore. That could be what's causing your issue, especially when you mention that you've rarely used that computer for a couple of years now. Plus, the newer versions of Bitcoin Core sync much faster than the older versions, and offer pruning as well (reducing the storage requirements for running Bitcoin Core).