Strictly on this sale, there's
Trials of Fire. I haven't played it yet, but RPS had a pretty good
review of it, and the Steam reviews are likewise pretty positive. Not exactly sure if it's a deck builder or not though.
There's also
Etherlords II, which is super old, but basically a mix of MTG and Heroes of Might and Magic. I liked it when I played it, but that was admittedly something like 15 years ago, so I have no idea how good it is compared to more modern games.
Despite being primarily multiplayer,
Faeria also has a surprisingly large amount of singleplayer content. It also has really great and creative gameplay, which helps set it apart from other games in the genre. Purely in terms of the actual card gameplay, I think it's one of the best out there. Certainly one of the more creative ones at least. It used to be free, but they started charging for it a while back. And it also has a bunch of expansions, but I quit before they really started pumping them out so I don't really know if any of them are needed. Sadly it is still primarily a multiplayer game, and that part of it is kind of dead these days.
For games that aren't on sale:
The Lord of the Rings: Adventure Card Game might be worth keeping an eye on. Right now, I don't really feel like it's good enough to recommend, with its biggest flaw being the lack of content (which will presumably be fixed in the future), as well as a pretty sucky pricing structure (which is why I'd really recommend waiting for a heavy sale. I just don't think there's enough value's in it at full price).
There's also a bunch of free stuff.
Card Hunter is great, with a singleplayer that's completely fine without paying. It's super dead, but I guess that shouldn't really matter if you just want to play it solo.
Hex also has a pretty meaty free singleplayer campaign. I didn't really play that much of it simply because it just felt like an inferior version of MTG (and the Microprose MTG game is a far superior singleplayer MTG option), but it is an option if you like MTG's gameplay.
Also just as a note, I'm pretty sure that the Steam version of Thronebreaker does not give you Thronebreaker cards/rewards in the multiplayer version of Gwent (only the GoG version does). Which would be a pretty significant drawback if you ever want to play multiplayer.