Joint taxes and same health insurance are strong. Receipts and photos are lesser evidence, but include what you have.
Go through everything that you can get your hands on, physically and digitally. Take every piece of evidence physically coming in that indicates your shared life together (even some junk mail), and throw it into a box. Do the same thing digitally in a computer folder. You might surprise yourself with the amount of evidence to support the filing.
Notarized affidavits from people who know you are for when other evidence is weak; they don't need to say much except how they know you and for how long, and that they've observed and are attesting to your continuous marriage. Omit details about problems. Each affiant should list complete contact information and their relationship to you. Statements from prominent people in the community (officials, pastors, et al.) carry good weight.
For most couples, I recommend shopping for a lawyer who can draft wills, medical & financial powers of attorney, and similar estate documents at reasonable cost. These are superlative support for the I-751, and they're even more crucial to have in "life." If you've had relationship trouble: At minimum you should have medical powers of attorney and HIPAA releases, naming each other; these should be safe to have should your relationship crater otherwise. Look on your state's website to see if state-compatible templates exist for you each to fill in and print out; if not, shop for a lawyer. From observation, I do not recommend "doing it yourself" otherwise from random "legal" sites on the Internet.
With the above in mind, let us know how you choose to proceed, and keep us informed otherwise.