Most newer seats allow you to use all harness slots for rfing. And the Triumph is the only one I know of that has that huge "jump" in distance between the rfing slots and the ffing slots. I had an old triumph like that for my oldest (he's 8 1/2 now), but he was a big baby with a monster torso and outgrew it rfing before a year!
On the upper end of your price range, you could look at the Diono Radians. They are usually closer to $200, but sometimes you can find them on sale. They go to 45lbs rfing and have a nice, tall shell. They do convert to a booster, but they are outgrown in booster mode really early (usually by the time the harness is outgrown), so you will need to buy a dedicated booster later. That being said, the seat should last him to booster age, so after the radian is outgrown (which ffing could be anywhere from 6-8 years old), you just have to buy a booster and be done with seats. One issue with the radians, is they tend to have quite a recline rfing, so you would probably want to buy the angle adjuster when he's 1 year old to make it more upright easier.
I've heard good things with the Graco Size 4 Me. It tends to be around $180. I also own and like the MyRide (which is closer to the $150 range - Target has a couple of them on sale for $135 this week), but depending on torso height it might not get your son to booster age. My boys were tall, and wouldn't have made it, but my daughter is 4 and still fits with about an inch of growing room left rfing. It will last as long as most of the Britax seats would, for about half the price. Those all go to 40lb rfing. If the nautilus you have doesn't expire for a few years, you could use it for a year or so after the myride until he's ready for a booster. There are other Evenflo seats (like the triumph advance) which might work for you, but if you don't like the one you have now, you might want to just avoid those. The True Fit is another seat to consider, they run around $170, though you can look for sales and get them cheaper. The True Fit goes to 35lbs rfing. Another seat I love, but some do not is the safety first complete air. It goes to 40lb rfing, but the manufacturer put a 40" rfing height limit on it. You can make a parental decision to go beyond that, or not, it's up to you. Just for reference, my daughter at 4 years was just barely 40", so in all likelihood it will last your son somewhere between 3-4 years rfing. It has a no rethread harness, which I love, and air filled padding around the head. My oldest, with the long torso, still fit in it ffing, just barely, at 6 years old. The average kid would probably last until at least 7. So definitely booster age.
Probably your best bet is to go to stores near you that carry these seats, take a look, and see what you think. Then you can google and find the best deals online if you don't find a good deal in store. If you plan on your ds being your last, you really don't have to worry too much about getting a seat that's a year old and a year closer to expiration if you can get a really good deal on it. If you want to pass it down to a future child, I would make sure to try to get one with a more recent manufacture date. Albeebaby.com is often good for deals on discontinued patterns for seats (i.e. last years models, and the newer seats have different covers).
I hope that helps!