You'll be able to do some money moves to take advantage of your low tax rate when spending your accessible money. Things like converting pretax accounts to Roth accounts can be useful down the road as they'll make your taxes at 60+ lower. Roth accounts don't have RMDs, so if you're planning on having lots of assets once retired, it's useful to have your balance in a Roth account.
I've heard too may stories about funds in a checking account going missing to trust that much money in an checking account. I would keep any cash funds you want on hand in a money market account or savings account, and move a few weeks or a month over at a time. You don't want your credit card company pulling out $29k instead of $2,900 and leaving you scrambling for funds.
How much to keep in cash is usually how much makes you conformable, but any money you keep in cash instead of invested affects your asset allocation. So account for that in your projections, or simply subtract that amount from your nest egg in calculating if you have enough. There are some withdrawal methods that rely on keeping an amount of cash on hand that you spend on poorly-performing years, but it doesn't sound like you're asking about that.
