This is very wise of you and your wife to get your finances in order. This will help out in many areas of your life but most importantly, your marriage!

1. LLC vs. sole proprietor needs can differ depending on the state. Typically, forming an LLC offers you a layer of extra protection and "official" status to your business and personal lives. You can also "pay yourself" through an LLC which can help manage your business and personal finances a little more and at the same time, lower your taxable income.
2. Going with an LLC is never a bad decision. Regardless if you decide to go separate, you should probably ask the IRS for an EIN number (you can do this without an LLC).
3. Definitely have a separate bank account, debit card, credit card etc. for your business. This should be separate from all of your personal stuff.
4. AMEX offers a decent 2% cashback on most expenses so that could be good option. There are many great business credit cards to use however. Capitol one has some good products as well.
5. Bank accounts SoFi, Ally, (Sofi and Ally are super user-friendly) Discover all offer around 3.5-4.00% interest in their savings accounts (conditions apply)
6. 25k is a good start. It's never to late to earn some extra interest in a HYSA. It would be a wise move to move that into a HYSA. Credit Unions can be a great choice as well if you have a local branch that pays a decent interest (3% or higher in today's economy should be good).