Nia is a bit of a longterm goal. You can get 2 stars super easily for the first skill potion, by just doing the quest after the main story and buying the 2nd one for 8000sun coins(hope you've been saving chests). The quest takes a while but basically an hour after you finish Punika you can be done with 2omnium stars for 6skill points. The 2nd skill point requires all 6 omnium stars, and one of them drops off Moake, which is the world boss in punika but requires 1430 gear to kill, which realistically isn't happening for a few weeks at the very least, and likely a month or two, unless there's an easy way to kill him I don't know about(you can zerg one of the T3 boss that spawns on a small island as you respawn right next to the boss when you die, but the other stuff not so much). You do get a legendary Wealth rune at 5 omnium stars though which is nice if you play any class that builds ressources, so I'm still working on Nia somewhat but yeah shouldn't be a big priority, and likely you'll want to skip most of her rapport by doing a big gift at once, since she also has high virtue stats reqs(330 wisdom/kindness once you get past friendly), so you only really need enough to get to the part that sucks.
Personally I've just been saving gifts for now but for my daily stuff, I've been completing the easy charisma pot ones, Elia in East Luterra, Azena in Rohendel(only need first reward, not the whole thing) and working on Yuul atm, as Charisma is the hardest stat to come by in my experience(I'm almost 100 behind the others on charisma even though I've farmed these and Una rep specifically). I'm also doing Beatrice since she has both a charisma pot and a giant's heart but that one takes a while. For the rest, atm I'm working on Blackfang for the Astray eventually. I do 2/2 on Beatrice, 2/2 on Blackfang, 1/1 on Yuul and 1/1 on Nia. I also did Seria before(10days for Rethramis adventure tome food), worked on Thunder for a bit(30days for Yudia tome food, but stopped working on her atm as I'm not gonna be doing adventurer tome anytime soon anyway was just a filler), Ryeon's Ghost(10days for courage pot, was fast and easy when I was leveling/early).
As mentionned, Sasha/Calvasus/Beatrice/Nineveh are solid picks since they give giant hearts, but they all take a long time, Beatrice being a bit shorter since the heart is a reward earlier for her. If you get all the other hearts, you only need 2 of them to get 12 hearts though, so you should do Beatrice manually and gift one of the others once you have 11 hearts(many other hearts are also time gated, Tooki boss una task for 30days, Azure wind island una task for 15 days, shadowmarket una for 5days). If you P2W you can brute force them all, and I guess if you buy gifts constantly for blue crystals on mari's shop you might also be able to do it even without P2W. Personally I'm just waiting for tooki task to be done before I really look into 12 hearts, although 10 is easy enough.
If you want gold, the shit in Feitan and Punika reward stupid amounts really fast. There's a squirrel in Punika that rewards 1000g in like 10days of just doing the emotes/songs. I haven't looked into the conversion but pretty sure on most of them you gain money if you buy blue crystals, buy Mari's shop gift boxes, then gift these guys.
On skill points, also look up the guide on maxroll, a few are time gated after various stories. Rohendel needs you to do 7 days of Una Tasks there to get the potion, Feiton takes 4 or 5 days of doing daily quests in there(just normal quests that pop every day, post story kind of thing), same for Punika 5 days of post story quests. Punika also has 21 days of una task for a skill pot, while Whispering Island has a skill pot for 10days of una task. The earlier you start these the better, skill points are great(although technically there's breakpoints so it doesn't feel very rewarding inbetween those).