Easiest way not to be denied? Submit all required documentation as soon as possible after applying. Prepare your application and you can easily upload it all the same day you apply.
Relationship evidence for a 300 is not the same as de facto/spouse partner visas. My partner and I had no joint account, no joint finances, had never lived together (we don't count his short holiday visits as officially living together even though he stayed in my apartment), didn't share a lease, and had no joint ownership of anything. We clearly lived in two separate countries and had a long distance relationship. The combining of things came after the 300 grant once I migrated to Australia.
The key is to prove you've met in person at least once since turning 18, upload a NOIM or letter from a celebrant to prove you intend to marry, and give what proof you have of your relationship being genuine.
Of course you'll need the other things on the checklist like FBI check, state checks for any state you've spent 3 months in in the 12 months months prior to applying, at least 2 F888s, F80, background check for your sponsor, etc.