What if someone found the key and spent the btc to another wallet ( i know, pubkey...) but with a super high tx fee, like extremely high.
Which key? puzzle key? 66?
If the 66 key is spent it may enter in some bot fight each one will increase the fees compared to other until there is nothing left to spent but the final result is unknown
Not always the higher fee wins, chek my previous reply to this:
Just to mention that when the nodes have many FullRBF transacions not always win that one with more fee, here some examples, Dot with Green margin was mined, some are Testnet and other are mainnet

Here was mined a TX with 1 sat/vB instead of a 37 sat/vB

Here was mined a TX with 22 sat/vB instead of a 44 sat/vB

Here was mined a TX with 106 sat/vB instead of a 1032 sat/vB
All depens of what is the transaction that the miner selected to work with his hardware.
I tried to import a private key into bitcoin core but i got this error :
Only legacy wallets are supported by this command (code -4)
what should i do now?
If you don't show what command do you used, then there is no way to know what is wrong.
What kind of address are you trying to import? it looks like it is not a legacy one.