I need advice on how to spend gold

I suggested arena in my post, as did the next poster. One caveat to that. If you can’t average 4+ wins per run and/or don’t really like arena, then you’re better off just buying packs with the gold. If you can manage 4+ wins, arena effectively gives you discount priced packs.

I personally average 3.75 wins per run, but since I like it, I don’t mind that I’m not really getting a discount, but pretty much breaking even on the pack prices. And of course, the more you do it, the better you get, so eventually I expect arena to give me a discount.

Spend all your current gold on classic packs as those don’t go away. Don’t bother with previous expansions. I you eventually need a card from these expansions, craft it with your dust.