It looks like kwallet takes more setup than I remembered, did you try following section 2.3 in the wiki page?
The other thing I can think of may not be relevant (been a while since I’ve used plasma now) but I’ve heard plasma has a built in greeter these days so maybe worth double checking to be sure you’re really using sddm. I feel like if it’s installed it’s safe to assume you’re using it, but if you’ve tried everything else this might be a worthwhile sanity check