I, like you, was extremely, extremely concerned that any tiny minor mistake would result in a denial. That is just not the case! Agencies like Boundless like to fear monger a little bit to convince you that you need their service. My now husband and I also almost went with Boundless for that reason, until we found this forum and realized how doable it all is. As others have mentioned, though, the only requirements for a K1 visa are proving that you have met in the last 2 years, and that you are both legally free to marry. Any other doubts they may have can and will usually be settled by an RFE. As long as you prove those two things, don't stress about getting rejected - it most likely is not going to happen to you!
Now, even though others in this thread have said photos and evidence of relationship isn't necessary (which is true) I did elect to include some in my packet, as well as a letter detailing the circumstances of our meeting online and giving a timeline of all the visits we'd had to one another. (The letter was also meant to introduce/supplement the many boarding passes, passport stamps, and I94 we included.) We had been together for 9 years, so I actually felt an opposite pressure to you - explaining what took us so long! 😄 Again, none of that was necessary, but I was anxious and it made me feel better, and ultimately it didn't hurt.
To your point about the consulate finding your 18 month relationship odd - if you peruse this forum, you'll see many K1s that were approved with much shorter relationships. It might not necessarily be socially typical in your circles, but consulates look at things a bit differently, and in this day and age they are very used to seeing online/long distance relationships. I don't see that raising any red flags at all, as long as you can prove you've physically met within the last 2 years of filing.
I totally understand why you're anxious, but truly, it sounds like you don't have a lot to worry about! You're at a low fraud consulate. Just follow the rules and the worst thing that will happen is an RFE!