is there any reliable solution to import the wallet to either sparrow or electrum, and get a seedphrase that way?
You cannot import Bitcoin Core's wallet file into another wallet. And while afaik the new Bitcoin Core wallets are also HD, they are not based on a seed in the way you see it for Electrum or Sparrow (BIP39 and so on).
Is having the seed that important? The private key(s) aren't enough for your needs?
If you want to obtain the HD seed of your Bitcoin core wallet (but it will be BIP32, not BIP39), you may want to
read this on Reddit and maybe
try your luck with this tool (but I have no idea if it works and I would not use it with a wallet that may have anything valuable in it).
So, as I said, maybe you just get the private keys of the relevant addresses, it should be easier.