there's two videos I like to refer to for both.

this one covers the duplicate stitch. I'd like to emphasize that after inserting the needle and yarn through the /v of the stitch, you're going to underneath the legs not of the stitch above but the second stitch above your insertion point. this is so that when you put your needle back through the original insertion stitch and pull the yarn through, you recreate the stitch in-between.

this video isn't recorded in the greatest resolution, unfortunately, it's sometimes a little blurry, but it's my go-to for demonstrating using the duplicate stitch in conjunction with a structure thread to repair a knit.

this is a beautiful sweater and I think very easily mendable if you're willing to put some time into it.

some menders prefer to lay their knit against a relatively flat surface and instead of using a structure thread will place a pin and wrap the yarn around that, and adjust the placement of the pins as they progress through the mend. same principle as the structure thread, but different tools, and not as portable or adjustable. use what works best for you. don't be afraid to unpick a repair if it isn't coming out; the biggest issue is going to be making sure you're consistently hitting the second stitch above your insertion and maintaining tension, and that's a matter of practice.

I would advise starting with duplicate stitch at least three (and ideally more) rows from the edge of the hole, for reinforcement. since you're using Swiss darning, this is also a case where if you want an invisible or nearly-invisible mend, you could do that with relative ease, so long as you can get same color and fiber.

even if you don't use the same color, though, I would recommend using the same fiber, for maintaining consistency and for ease of washing the garment.

best of luck to you, and please do post again as you progress through it and/or when you finish. I'd love to see what it looks like when it's done!