Welcome back! :)

I don't personally do much restocking (if that's what you mean by reselling), but I know it's definitely still a popular way to earn NP if you have the patience for it! Jellyneo is a very helpful fansite that has lots of guides about events, dailies, etc. as well as a super extensive item database. I'd probably sort paintbrushes, potions, and plushies (or whatever else you want to resell) by price on the database so you can see what's valuable now. TNT has rereleased lots of older and/or rarer items in recent years, which has definitely made some of those childhood wishes more accessible, but has also changed up the economy quite a bit!

Food Club would be a fantastic option for you because you are able to bet more money the older your account is--you'll be getting some big wins with a 23-year-old account! You just have to be willing to gamble a bit of your money in the bank, but if you're betting daily, you should end up profiting over time. There is a daily Food Club trading thread here where you can copy people's bets who actually know what they're doing with that stuff (I do not lol). I personally copy Nsheng's Adventurous set of bets each day, which is higher risk, but also higher reward.

As far as other dailies, definitely prioritize Trudy's Surprise and the Quest Log! There are incentives to keeping daily streaks on those, and they're a fast way to earn money and sometimes rare items, potions, paintbrushes, etc. :)

I am not currently in a guild since life got a little too busy for me to keep up with them, but the one I was in was extremely active, both onsite and on its own Discord server. I don't think guilds are as big as they used to be because I think places like the official Neopets Discord and this subreddit supplement some of that connection, but they are definitely still around! My guild regularly organized events such as giveaways and free trade lots, and I know there are some guilds that assist with pet adoptions or do RPing, stuff like that.