Letters written to one another and physical gifts are not strong evidence when you consider the totem pole of evidence.
Visits are definitely strong and contact with family members can also be beneficial. Now, think about it and don't box yourself in.
For example, we (my husband and I) do not use social media. Therefore, we did not have all the posts about one another or the interactions with the family. However, the trips that I made there, I took one or two pictures with my mother-in-law and my sister-in-law (they hated pictures, but we made sure we had them). My parents are older than the average age for our generation. Thus, their contact with my husband while he was in Argentina was not constant because they do not use tech-savvy phones, but they did talk on the phone and they were aware of our progress. What we did was include signed letters of support from each parent sent stating they knew about the relationship, how it progressed, that they supported the relationship, and that even though time spent was limited because of distance we all were like a family unit. The letters were signed and included with a copy of a picture ID of the parent that signed it to corroborate the signature. These were included in the similarity of the intent to marry letters (we went the K1 route).
As far as the travels and the meetings, maybe frequency is not an option but longevity could be. Shorter trips more often might not work for expenses or whatever your plans are, and that is OK. But, maybe she can come and work remotely for a certain amount of time? In other words, 4 trips a year for 5 days is frequent but not enough time to co-exist versus one trip a year that is a month or longer would give you time to get to know each other on a daily basis and it will also be good for USCIS because it is more time spent together (i.e one month or 30+ days is more than 4 trips of 5 days, which is 20 days). I had longer trips myself where I would stay 3 or four weeks, and that was better money-wise, for our relationship, and for USCIS. In fact, my last trip was purposely booked once I knew the interview date. I booked it so I could arrive a few days before, be there for the interview (even though it is not needed and I was not asked to come in), and we had plenty of time after the interview to finalize apostille and legalization prior to both of us leaving the country together.
Same sex would be an issue in other countries. But, Argentina and USA are not one of those countries. Regarding the age gap, by itself, it is not enough to cause a decline, and there are plenty of couples on VJ that are both same sex and have an age gap that have been approved without any issues.
K1 and CR1 have both their pros and cons and one you can decide what is best for your journey and where your relationship is at. @Crazy Cat has a good list for each option that can help you make that decision.
All I have to say is, immigration is going to be tough, regardless of the option, and it will be a long ride. Make sure you both talk about it, consider where you are at, what matters to you most, your needs as a couple, and manage your expectations. Then, buckle up for the ride.