When I first started, I'd end up running the gift daily a lot because I needed an inefficient way to burn off my stamina so that I wouldn't waste any, and early on you easily level without stopping from your full stamina bar + 3 gathers. The normal missions and beginner garden are too exp efficient early on, and I'd end up leveling up with extra stamina left in my pool, which is just wasted. Once I hit the point where I needed my full stamina bar + the 3 gathers stamina to level, I stopped doing the gift daily, and haven't touched it much since. I'd agree it was a nice boost early on.

I got most of my gifts from farming events. Though I don't do it much now because of stupid stamina costs.

Depends on if you want to gain all the content in the game or not. I'm at 30+ girls 100% right now and have started on 2* girls as I have no more 3*+ with a certain gift type to lvl affection with.

It's worth getting 100% for your 4*+ girls if you are after game clear only but don;t bother with 3* or less as you will replace them all with 4*+ soon enough.

But if you are a pokemon style player then you will want those gifts just to unlock all the things.