Sorry for the late response, been knocked out by covid for the past week. Hm, looking at the cloud wallet interface, there should be some US-available exchanges for it, both fiat and crypto options.
Level 5 rare summoners, yes, that's where I've been since Beta. Right in the middle between silver and gold, too strong for silver, a bit too weak for gold. I see you got your Tarsa and Thaddius up to level 6 now. Good call. Now they're maxed out in gold league. So just the rest of the collection remains! I think you're doing it in the right order, slow, steady and patient. And buying singles, not packs, that's the way to go.
Baakjira at level 3 IS amazing. In semi-low-mana battles with the Noxious Fumes ruleset, I put him in the back, while the rest of our teams fight it out. Enjoy, adversary :D Another interesting card to look out for that's super cheap now, is Pelacor Conjurer. Quite strong in low mana battles