I start geth 1.8.16-stable-477eb093 on Windows 10 with the following command on mainnet:
geth --syncmode "fast" --cache=4096
it starts to sync, but currentBlock never riches highestBlock, although they are close and the difference is about 100. eth.blockNumber is always zero. I have the similar situation as described here, but on the main network.
geth 1.8.20 starts syncing, but crashes.
I waited for a couple days, should I wait more?
see my blog post for more details.
Even though it crashes, the next time you start the geth client again, instead of starting from the genesis block download, it will just continue from where it left last. So, yes keep continuing to download. Also, just FYI, you don't need to be exactly in sync with the current block number. The only caveat with this is, whenever you perform a transaction and what to see in your geth instance you have to wait for your syncing to reach to that block height. But again, why would you want to go through that pain. I would suggest use Infura which is free or better use Quicknode (though it charges a little).