1. I say hold off as long as you can because at this age, it really IS within your control. We've made mistakes 2 yrs. in a row now.
Ds' 3rd birthday was his 1st party. We had it at our house b/c I was nostalgic about homemade cake & food, casual, etc. We did rent a bounce house so they would have something to do.
Then came the realization that with both of us working, we didn't have time to make food. I killed myself cleaning the house from top to bottom, & we ran around like mad pulling it together. The guest list grew much larger than we thought it would b/c at this age, siblings & parents come. All of a sudden, we couldn't even do the cake or cupcakes b/c it would be too much. We ended up having to buy food & cupcakes, & rent tables & chairs. And then hope fervently that the weather would hold so that the party could be outside b/c our house is too small.
The weather held & everyone had a good time. Ds loved it & still remembers it, but it turned out to be expensive AND exhausting. Lesson learned, at least for us. No at-home parties that extend beyond family.
This year, we debated what to do, if anything. B/c he remembers his previous party & has gone to a few through preschool, we thought we were stuck, so we planned one at one of those kiddie party places. As it draws closer, we realize he really wouldn't have been scarred for life if we hadn't planned one. He's still too young to really figure it all out, so we could've done something very, very small.
We have friends who live for this kind of thing. One still gives me a hard time for skipping #1 & #2. So I think it's fine to do it, but we really don't live for this kind of thing, so we do wish we had made some different decisions. Hopefully we'll find the right balance with #2.
2. Not opening presents at the party seems to be the norm here. We only have 2 hrs. at the party place & they charge $50 for every 10 min. you go over your time. Yikes! Plus it eliminates any awkward situations or rudeness that can come from small children opening gifts. I was really surprised when I first heard of this trend, but now I'm relieved. It also allows the parent to track who gave what, if they write thank-you notes.
3. I don't think thank-you notes should be over-thought in terms of before or after. Thanking the giver for the gift is what matters. I write them when I write them, often over a period of days. If ds has played with the toy, I say how much he is enjoying it. If we haven't tried it out yet, I say how much he's looking forward to it & I know he will love it. Honestly, it never occurred to me that anyone might think twice depending on the version they got.
Good luck with what you decide. I do think people go overboard on parties for small children & I'm not super proud of my own choices so far, so the best you can do is try to stay true to what feels right for you. (Says the one with party regrets!!)
(And don't get me started on goodie bags. I absolutely hate the practice with a passion. Yet it is the norm here, so we have done them. I still hate the practice, though, & yes, I know hate is a strong word.)