Ah, well first of all it sounds like you are doing EVERYTHING RIGHT. Leaving awesome comments is the first step towards a happy fic writer. As is talking to them. Both strong wins. If you are willing/able to do more, here are more suggestions, or as I like to call it:
“Ways to Make Fic Writers Blush”
1. Comment On the Fic (you already do this but just to make the list pretty and comprehensive)– Any comment is much appreciated. If you have time, leave a long one. We live for long comments. Or comments where you say you are re-reading the fic! Repeat commenter comments!
2. Send them a Nice Message on tumblr
3. Recommend Their Fics: make a rec post on tumblr, be detailed with why you like the fic, tag it and link to the fic directly! Include their fic in lists like “Best Fake Dating Fics” or whatnot! Or just recommend them to friends!
4. Make an Edit: Yes, your instincts are ON THE MONEY with that one. This does not need to be complicated or super time-consuming–any bit of Creative Output that was inspired by a fic is AWESOME (though do remember to link back to the original) (And my goodness, PLEASE TAG THEM, the tumblr tagging system isnt perfect but it can help And every once and a while, i attempt the search feature only to see things are there for me and i hate that I missed it the first time!)
5. Fanart!! Warning: if you draw something for an author, they might die of joy.
6. Message the author a copy of a conversation where you talk about their fic: This is oddly specific but one time someone copy and pasted a conversation they had with their friends in a group chat where they were ALL TALKING ABOUT MY FIC AND JUST LOVING IT AND YELLING ABOUT THEIR FAVORITE PARTS and I actually did die. I am writing now from beyond the grave. It was incredible. (Note: ask friends before doing this)
7. [if its a tumblr fic] Reblog with nice tags (authors read the reblog tags. for real. promise)
8. [if its a tumblr fic] Reblog and tag your friends in the comments– knowing that you want your friends to read the fic badly enough that you are tagging them is awesome!
Hmm… that’s all I can think of for right now. Other fic authors feel free to add!
Thank you very much for this question, it was such a sweet one!