I downloaded the God’s Unchained client a while back & played a match or two. I should really revisit it further. I also enjoy that Magic/Hearthstone style gameplay better too. I think Splinterlands lack of that format as an offering has hindered its growth.
I personally haven’t found expansion cards in Splinterlands to be necessities, my collection certainly skews heavily toward beta cards. I think once you get a feel for the various randomizes rule sets and some of the new monster abilities you may be facing you’ll do a lot better than your first reintroduction.
Gods Unchained is a lot of fun for sure. They've recently added more beginner cards that come with your account, so you have more deck-building options against those who have purchased a lot of packs and may have some legendaries.