Just to clarify how Bitcoin works:
How can I retrieve it?
You don't ever really retrieve Bitcoin money, it is always kept track of in the list of transactions that almost every Bitcoin user has their own copy of (or access to a trusted copy).
This list of transactions is a transaction-journal that is called the blockchain, but it isn't owned or controlled ...
Bitcoin is secure when 51% of the miners are honest. We also need every node to have at least one honest peer to avoid eclipse attacks. Are these the only assumptions based on which Bitcoin is secure?
I don't consider them as assumptions instead they are different types of attacks.
51% attack is the most over-rated. You can find lot of Q&As related to ...
Generally it is advised that you don't bury hardware wallets for long periods of time as they need to semi-regularly have their firmware updated to address vulnerabilities that get discovered over time. Instead what you should generally bury is the mnemonic seed on metal (encrypted with a passphrase or "seed extension" if it is possible an ...
If you don't have your private key, and the custodial wallet that does have your private key is no longer accessible, then you cannot move your coins or "cash out".
Invest in a hardware wallet, and transfer your coins out of any custodial wallet if you plan on hodling the coins. That way, if you ever need to cash out, you can.