The level of security in maintaining investment must be based on the ability of each individual in managing and that must be a basic concern before everything goes further. People will never care how we save bitcoin in a place that is much safer or not and people never want to know how our involvement is bitcoin has gone through a right or wrong process. So every decision regarding investment must be carefully thought out and regarding the level of security of the asset will be an custody of itself that needs to be maintained.
It must start with their research for learning fundamentals of Bitcoin, including how to store bitcoin safely (in open source non custodial wallets with private keys/ mnemonic seeds for full control of their bitcoins), and where to purchase it before withdraw it to their self-custodial wallets (they can do it on CEX, DEX, P2P market places but at the end, their bitcoins must be stored in non-custodial wallets).
It's about research for fundamental but vital things before investment, and a next vital step is how do they manage their capital and related risk for investment. Invest what they can afford to lose, and invest with own money, no loan, no leverage, will help them having better capital and risk management, hence will have stronger hands for holding bitcoin in their investment portfolio.
Generally people will learn from mistakes that have been made but if they talk about assets that involve a lot of money, it should be more selective and not let something unwanted. Security priority is very important because it must be done in the right way and not left to run so.
Mistakes can be made by everyone and we all can learn from our mistakes, big or small, but it's very painful if mistakes will be paid by own money or most of capital we have. It's preventable if people are ready to learn and have good preparation for their investment.