If its just a GBA game the easiest way is just to use the switch. Theres no need to use the DS really. Even if you find something that uses the .sav file you still need to take the memory card out of the DS and switch (or use hekate). And copy the files over. Plus if your doing it all the time you might corrupt the memory card.I'm aware but I'm trying to keep all my save files synced between devices.
At the end of the day all I want is something that emulates GBA games and can load .sav files on the Nintendo Switch.
Change the format I can do but I'm trying to see if I can make it easier in the long run, that and because it doesn't hurt to try.
Renaming them isn't really much more work... and you can do it in batches. I'd probably think about leaving the Ds for DS games and moving everything over to the switch..... it'lll save ayou a world of pain.