Personally, I'm a big fan of the "this is fun for me" argument.
You asked for arguments you could make to convince your DM to let you be a grave cleric with your chosen deity. As a DM, one of things I am looking for to help create an engaging story is player buy-in. If the players have bought in to the story we are telling is both easier and more fun to tell that story. When the players are engaging with the story and with their own characters, it is significantly easier for me to prepare for the game and keep things fun and moving forward. Ergo, when my players are looking into character building and advancement, I am looking for the things that make them interested in the character they are playing.
What you are proposing here is a good example of something that makes me, as a DM, say "oh hell yeah!" You have a character concept you like and a mechanical selection that you like. When my players are hyped about both who their characters are and what their characters can do, that is the sweet spot I am looking for.
Now, the Player's Handbook and Dungeon Master's Guide offer some suggestions for pairing cleric domains with the gods of the various pantheons presented. The pairings there are loosely thematic. Very loosely. I see no reason to hold to those pairings as hard and fast rules. As long as the player can prove to me that they have thought about the pairing enough to be able to articulate the blended theming and how they work together, I'm generally cool with any cleric domain and deity combo.
So when making your case, just explain to them that you've thought about it a good bit, you're really stoked to pair this domain with this deity for these reasons, and respect their decision. That's the most important part - respect their decision. If that's a deal breaker for you, that's okay, find a new game. If you still want to play with this group, you may just have to compromise. How you handle it is up to you, but it is important to be respectful and to respect the DM's ruling on it.
Sarenrae's followers die too.
I'll also leave you with a little nugget of an in-universe argument you might consider bringing to the table: Sarenrae's followers die too. You mentioned grave cleric, and someone needs to be responsible for handling the last rites of the faithful dead.
Playing a cleric with a lively and active faith puts a lot of responsibility on the DM.
It may help to see things from the DM's side a little. Selecting a god as a cleric is a matter of how your character building relates to the DM's world building. Your choice of god needs to be sensible in the context of the world, and the DM needs to be okay with you interacting with the associated practical faith of that god. If your experience as a cleric is to have anything beyond just you saying "I pray to Sarenrae", then your DM has to work those opportunities into the narrative. If you interact with other followers of Sarenrae, the DM has to roleplay them. If you do any quests for Sarenrae, the DM has to build them. If you ever cast commune, your DM has to roleplay your god answering your questions. Playing a cleric with a lively and active faith puts a lot of responsibility on the DM for interacting with that aspect of your character, so you need to ask them if they are okay with that choice.