Money meant for expenses and money kept for investment are different. It is very wrong to spend on items from your investment portfolio, that is a bad idea. It is a criterion to have several wallets. One wallet should contain the amount of BTC you want for spending and the other should be specifically only for your investment alone. You can't be spending so much fees every time you spend from the same wallet you are using to invest.
I leave it up to you how many wallets you use for investing. I will not comment on this. But why would you save your expenses in Bitcoin? Is your salary paid in Bitcoin? Even if your salary is in bitcoins, do not save bitcoins to spend on daily needs. Split your earned bitcoins and convert bitcoins to fiat to spend on demand. If your salary is paid by fiat then only convert the money saved for investment into Bitcoin, the rest you save in fiat.
You can take different approach to investing, everyone has investment strategy. In this case you must be aware of the basics. For example: investing for the long term, being consistent, ensuring protection and managing investments properly. You can set up some funds to manage the investment successfully, but these must be set up by fiat. Never store essential funds in Bitcoin. Because you can suffer from it in your time of need. As you mentioned, "Bitcoins should be stored in a separate wallet for expenses", this statement is completely wrong and such practice should be quickly abandoned.