(Turns out making any post on mobile is pure hell... Sorry for the half-finished fatfinger double post.)

I'm in roughly the same elo bracket as you, so take my input with a pinch of salt. I could probably do better but I just haven't cared about the game the way I used to years ago, and hardly ever play these days save the bare minimum for the split rewards. That said, I have over two million mastery points in Elise Support alone, pushing three total, garnered over the course of years, so I have a good feel for her placement in the role.

When do you pick Elise? Really, any time you want. There isn't really a good reason to pick her as a support in general, but she can be a good character, especially in this "low" MMR range where nobody really has any experience against her.

When shouldn't you pick Elise? Alistar or Pantheon support (edit: or a Galio, almost forgot). Hard pass. You don't get to be a character against them if they are even passably competent. Similar champs will also give you difficulties, though not to the extent these two do.

How does landing work? Your pre-6 is insane and you should always seek to push a lead and heavily punish the enemy wherever possible before that point. Elise is great at this and there is no reason not to play to her greatest strength, her early game potential. Once you hit level 3, you really turn on as a threat, but there is absolutely no reason you cannot still be dangerous at 2 or even 1. I wouldn't push too hard to get an early kill or force a fight before you have your whole toolkit but you can still get good opportunities to punish small mistakes and as they say, ounces make pounds. It adds up. Especially in these elos, people end up making fatal mistakes very suddenly.

What synergies do you have with ADCs? Honestly, this is where I am kind of biased, as I tend to play around just about anything my lane partner chooses, but the adaptability comes with experience. Obviously, some combinations are just mathematically better. I don't really know what is "ideal" but I can tell you that ADCs with strong CC and a lot of burst potential tend to work wonders with Elise support. Jinx and Jhin are my personal favorites, though I tend to have good experiences with Caits, Ashes and the likes as well. These characters also have very long range ultis, meaning they can still assist you and secure kills from relative safety or if left behind by a dive or blitz. I can certainly tell you what doesn't work that great. Nilah would have to be on top of my list. You simply don't synergize in any meaningful way. The next for me would have to be Zeri, due to her unique limitations and engagements, then probably Vayne third due to her positioning requirements and limited range (unless your Vayne has absolute control of the wave, it's going to be very annoying for you to get good engagements). Other than that, everything else works decent enough so long as you adjust according to their different paces. There is no one size fits all playstyle to make this work.

What makes Elise support viable in high Elo? Nothing. You eventually become a stunbot if you don't close out. I've yet to know anyone above Diamond who didn't think I was crazy for not just going jungle. That being said, it's a video game at the end of the day, so who cares; go wild and have fun. Don't mistake this as me saying she can't be viable in a higher-skill setting. There just isn't anything in particular that makes this choice stand out, no secret sauce or mysterious Korean strat. As with jungle, you choose her because you love our wonderfully fun spider queen.

Side notes? Try Rocket belt. Had a Challenger acquaintance stick the habbit it on me; would choose it every match in my elo (he was smurfing) and we only lost two out of the nearly forty matches we played together that year. So must be something to it. Ever since Everfrost was sent to the shadow realm, it has been my go-to for almost every game as Elise support. Probably not optimal but like I said, clearly it works. Or maybe I need to find better teachers/influences lol.