1) How to generate a private key safely? Online tools are obviously undesirable as they might keep a back door on all keys generated. Similar reason for any library or packages even they operate offline (I cannot check if these keys generated are in fact following some predetermined sequence).
I'm not sure if generating private keys using a third party software and tool is safe enough because there is anyways the risk of getting scammed by saving the private seeds by the tool. The random generated private keys are much better and safe to use. you can also write the key in a piece of paper and put it somewhere safe.
Generally, in my own idea if you do have more than 10K worth of bitcoin, the best thing you can do is to buy a lager nano wallet and do not trust the software wallets because there is always the risk of getting inflected by malware. Also, people say don't put all your eggs in one basket, so I would suggest you to don't put all your bitcoins in one single wallet and try to use multiple wallets. For example, I do have a wallet for long term investment and it's untouched for the last 4-5 years, also have another wallet I use it here on forum.