Returned to Hearthstone after 5 years, what should I know?

I’ve returned to Hearthstone after 5 years (I boycotted every blizzard game after their treatment of Blitzchung) wondering what the best way to be a FTP player is now?

Is Arena still the best use of gold?

How should I get gold now? I see that quests have all changed.

Any obvious ‘to dos’ I should be aware of?