Re: Bitcoin core, can’t send my BTC (not in memory pool)

If the addresses you have in your wallet show balance and transactions, but those same address have zero transaction history on a Bitcoin block explorer, then that pretty much confirms that nc50lc was correct and you have most probably loaded an altcoin wallet into Bitcoin Core.

You can dump the wallet using PyWallet to see what the addresses are... The address prefix (first characters) should hopefully give you a clue as to the coin type...

Refer:
https://bitcoin.stackexchange.com/questions/19589/how-to-determine-what-type-of-coins-a-wallet-dat-contains

You can also use the "strings" method... Open the .dat in a text editor and look for "names" field. Refer:
https://bitcoin.stackexchange.com/questions/57122/have-a-bunch-of-wallet-dat-files-what-alt-coins-do-they-have-and-may-be-how-mu

Here example of addresses:
1NwUfBG7kKNKjtoTbj8557Ty8MYiGXYupV
1NzGg6KKApR6M5SKSWGjfScsokVZK68yEc
1P2SjcwEJ1hrgWPjdFRfNiDvEpq6DJ8S8A
1P5JGtjq65L7mcW5B2ESHe8RCqQfW5KNE6
1P5QtTVmTYyPwqGU8BHJxQyndbgQ3gFzK1

They all starts from 1, am I right that its btc?

Yes, those are bitcoin adresses, and you can put them in any blockchain explorer..?

None of them hold any balance though? I'm guessing that your bitcoin core wallet might still be syncing, and has currently only caught up until the moment in time when these adresses DID have any balance..?

Yes, any blockchain explorer.
I downloaded full blockchain twice and its always synchronized.