Don't rely too much on photos though they are good evidence. About 20 over the period of your relationship is enough (Think significant events, photos with friends and family, etc... not just a bunch of selfies).
Look for harder evidence to supply. It sounds like you'll have it, you just need to be creative. You can find plenty of ideas through older posts in the forum. For example, proof of those holidays (other than photos) like flight itineraries with your names and seats next to each other, hotel receipts, major activities done, etc. You should be able to dig a bunch of these up via email or rewards club history if you use them.
As far as messages go, you don't need to submit everything. A few important conversations over time (some people choose a few messages a month and compile all those into one pdf) if you have any but you only need these for time spent apart from your partner. If you've been living together for awhile, then you don't need to worry about it as much.
I would definitely include the letter from your employer about the living situation. Did you initially notify the employer asking permission for your partner to live with you via email or in writing somehow? That would be a good piece of evidence to date the living arrangement as well. You can also use joint mail received, or things sent to you to that address and things sent to your partner at that address. I always believe official letters to weigh more than things from friends and family (like bank statements, work letters, letters from the government, etc) but you can also supply things like joint invites to parties or weddings, holiday cards from family and friends, etc. Also licences, identification cards, bank statements, insurance, etc registered to that address from each of you.
If no joint bank account, find some other joint financial evidence such as major purchases made from your or your partner's account for something relationship related. For example, my partner and I have bought a fridge, a washer and dryer, and a car together. Even gifts bought for each other for special occasions can be evidence.
Do you have some form of insurance you share? Health insurance, car insurance, contents insurance, life insurance, etc. Are you or your partner listed as beneficiaries for each other, in each other's wills, listed as emergency contacts on work paperwork? Do you have joint to clubs, gyms, volunteer activities, social activities?
Evidence of your planning to get married is good as well. Think any written communication on it or deposits if you've gotten that far by the time you apply. We emailed around to get quotes and available dates so that can be used as well. Conversations with friends and family via text about it could be good evidence as well.
The list of possibilities are endless, really. There's a lot more out there. I suggest a Google search of any of the following:
De facto partner visa evidence Australia
And make a list of ideas relevant to your relationship. Even though you'll be applying for the offshore 309/100, the 820/801 is just the onshore version of that so evidence ideas from either visa category are going to be the same or similar.
Besides the above, everything else that is on the partner visa checklist on the immigration website: https://www.homeaffairs.gov.au/Trav/Visa-1/309-