You're in a situation that many people like yourself face: with the work experience that you have now and your educational background (Ivy), the only schools that are worth it to you are the top schools. Top 8 schools would be worth going, and top 16 a "maybe". But beyond that it's not worth it.
So yes you are choosing the range of schools that are probably worth it for you to go, and schools outside the top 8 will be a harder sell for you if you got in. I get it.
Just know that with this in mind, you have to go into the process with realistic expectations that you may not get in anywhere. Go for it - just be realistic. H/S/W are stretches - yes you'll be up against folks of a similar caliber but with even bluer chip resumes (and even they have had a harder time getting in these days), but go in expecting that it's going to be a roulette wheel and hope the ball lands on your number. Chicago, Columbia, Kellogg, Sloan and Tuck are schools where you have more of a shot - not safeties, but if you play your cards right (you do a good job on the applications), you should have a reasonable shot of getting in (but again not real safeties). As for safeties, you're looking at schools in the top 16 - NYU, Cornell, Yale, etc. but whether they are worth it to you or not is really a personal decision.
As for signing up, best time is when the applications come out in May/June. I really don't think it's necessary to sign folks up this early (it seems to serve the interests of the consultants much more than the applicants to sign up this early). Having done this for almost 8 years, I can tell you that the optimal time to complete a batch of 4-7 school applications is roughly 6-10 weeks. And most people peak at around 8 weeks. You will get to a point where the essays won't get any better (i.e you've hit your limit, your potential), and that usually happens after around 8-10 weeks. Beyond that, you will either burn out, or turn in on yourself where you lose perspective and start changing things for the sake of changing things - and oftentimes for the worse (because you've lost perspective on what made your essays good in the first place and you start to second guess everything). You want to work hard on these applications, but not work them to death either or it ends up being counterproductive.