I will answer your questions first, then post more below them.
From your experience how long have been able to keep your child in a convertable car seat? (age) My oldest rode in a 5pt harness until he was 7.5yrs old, 52" tall and 65lbs. He was in a Britax Regent, now in a Monterey high back booster. My 5yr old is now in the Regent, he is 47" tall and 50lbs. My 3.5yr old is rear facing in a First years TrueFit.
When did you get a larger car seat? (ie graco nautilus type) (age) My oldest was in a Cosco Alpha Omega, was too tall for it at 3yrs old, we bought a Britax Marathon. He move dto the Regent at age 4.5 when our youngest outgrew the infant seat and moved to the Marathon.
Convertable or booster for a 3 yr old? I would go with a higher harness weight seat like the Britax Frontier or Graco Nautilus. Harness now for a couple more years, then booster.
What kind do you have? We have a Britax REgent which will harness to 80lbs. They have discontinued that seat in favor of the Britax Frontier, whic hwill harness to 85lbs and then booster to 120lbs. We also have a Sunshine Kids Monterey high back booster for our 8yr old, and a First years TrueFit (convertible) for our 3.5yr old.
How easy was installing? As long as you read the manual you will have no problem installing a seat properly. If you ahve problems, get help from a CPST at www.safekids.org.
strap tightening? Again if you read the manual you will be able to do this easily and properly.
Strap raising? Most seats need to be re-threaded to move the harness up or down. Some don't. Overall you need to do this very seldom so I wouldn't worry about if you have to rethread or not.
good head support if they fall asleep? If your seat is properly installed for your childs age you shouldn't have head flop.
Big or bulky size? Unfortunately, car seats are big and bulky. Choose one that fits your car, your childs age/height/weight, and is within your budget.
OK! So now that I've answered your questions, here is my suggestions. I would move your 1yr old to the Boulevard REAR FACING! Please google EXTENDED REAR FACINg for more information. My 3.5yr old is rear facing in a First years TrueFit. She is 36" tall and 30lbs. Perfectly comfortable. The AAP, NHTSA and Safekids, and all car seat manufacturers, recommend children remain rear facing until they outgrow the rear facing wight limits of their car seat, or at least 2yrs and 30lbs. The Boulevard is 35lbs and until there is less than 1" of hard shell above the head.
For your 3yr old, I would purcahse a Graco Nautilus or Britax Frontier. Magicbeans.com and albeebaby.com are having a Britax sale right now. You do NOT want to put your 3yr old in a belted only booster seat. Its not safe, not wise, and yo ucan find seats out on the market now that are a better choice for a small child.
For a rear facing car seat, the harness should be just at or BELOW the shoulders. The harness should be snug as a hug and the chest clip level with the armpits. A rear facing car seat is outgrown when the child is over the rear facing weight limit or there is less than one inch of hard shell seat above the childs head.
For forward facing, the harness should be just at or ABOVE the shoulders. The harness should be snug as a hug and the chest clip should be level with the armpits. A forward facing car seat is outgrown when one of three things happens; the shoulders go over the top harness slot, the tips of the ears are level with the seat back, or the child goes over the weight limit.
Bulky winter clothing should NEVER be worn in the car seat. In a crash it will compress and make the harenss too loose for your child and subject him/her to being ejected. Pleae read the manual and dress your child in layers, fleece, and put a blanket over or the jacket on backwards after the car seat is buckled. Or go to youtube.com and search 'car seat jacket trick' for a video on how to safely wear a jacket in the car seat.
And again, if you need help installing or using a car seat, please find a CPST in your area.