Struggling with getting a couple of keys the way I want them

[somnomania]

So I'm here from SwiftKey. I've got most of my preferences sorted out, but the one thing I'm missing from SwiftKey is the . key having ,!? on the press and hold options. Ideally I'd love to have that and then remove the dedicated comma key, or not, that part doesn't really matter as much. I've looked over the stuff about custom layouts and it's just not clicking for me. If I do a JSON file, do I have to specify every key in the layout or would it just apply changes to the keys I mention and leave the rest as defaults?

[Trasd]

I would copy the default layout and modify only the key you're interested in, leaving the rest alone.