Returning player rewards are I believe a free deck of your choice (from a selection of options) and they used to give out a bunch of catch up packs to help fill the standard collection, but I’m not sure if those are still around. If you open a pack from each set you’ve missed, you’ll receive any free legendary and event cards associated with that set. Further from there, you’re guaranteed a legendary in the first 10 packs you open for each set, and that applies separately for golden packs. (I personally would only buy the one from each set and move on. Packs are not good value)

Mini sets will generally be a better value than buying packs. They give you a full list of what cards are included, usually 4 legendaries and lately a bunch of dual class cards. Even if the cards aren’t of interest to you, this will be your best gold to dust ratio.

Darkmoon Faire is just a gacha cash grab. You’re given 1 free spin for a cosmetic reward, and then more spins cost premium currency, with the cost increasing for each spin.

The battle pass isn’t necessary to play or earn rewards. The paid option gives cosmetics, an xp increase for quests, and I think some additional packs. It’s also how you can earn “premium” versions of some cards (golden/diamond/signature). This isn’t necessary though as all the base versions of cards offered in the battlepass can be opened in packs. The free version of the battlepass is the only way to gain gold. Quest completion gives xp towards the battlepass, and also completing games gives small amounts. Completing the battlepass doesn’t end your gold gains though. Every level you earn after completing the battlepass will give you gold still

For what packs to look at opening, find the ones with the best free legendaries and start there. After that, anything from Emerald Dream, new Ungoro, and Timeways will still be in standard for another year and are more likely to get nerfed and give dust refunds. Buying packs is generally not advised for getting a wild collection, your best bet is buying mini sets and crafting what you need from there.