I plan to get my named changed at a point soon. Is it possible to use my name as it will be in the future, or should I use my current legal name? And if so, can I change said name upon legal name change?

You can use any name for your developer account and  Itch.io allows you to change that name. There is a link next to the name: Change my username.

You can use your future name as your display name, or you can create a unique name for your game business.

You need to use your legal name for the tax declaration.  If you officially change your name, or start trading under a business, you'd need to contact support and ask to retake the tax interview at the time you qualify for a payout (if you're using the method to let itch.io collect the payments on your behalf).