Question extra seat for 3rd car

i'm looking for a 3rd seat for a 3rd car (babysitter, school, etc.) and should easy to install/uninstall for many cars. i've only had britaxes MA/regent but don't want to spend that much money and want something more versatile that can go from harness to later, booster of some sort. budget is about 100-200 and may consider going over $200. i have 3 kids and 2 cars (2002 accord and 2004 pilot) and now 3rd car so someone can pick up my kids after school, etc. that's a lot of carseats that we use on almost daily basis so i don't want to have to switch carseats all the time . he's currently in an old alpha omega (babysitter's) that was purchased in 2003 so will expire soon. the seat will be for a almost 5 years old, 38-39in (haven't measured him lately) and weighs 38 lbs. and in a pinch may use it for my my almost 3yo who is 27lbs and 33in(?). he usually rides in a MA in babysitter's car. i would like a 5pt harnessed seat to as long as possible... and in my case, really long b/c my oldest is on the smaller side. side note, my DS1 is so used to having seatbelt on tight that he freaks out if the seatbelt is not just on but snug.

i like the frontier (will get for our car later) and am a diehard britax fan but want to see other seats that will work too that's a little cheaper for the 3rd car. i looked at the safeguard go and nautilus. nautilus i'm not so fond of b/c it's not so cushiony but it's for short trips to and from school so it's not critical. i liked the safeguard go b/c of it's portability. however i'm concerned about the safeguard go b/c it needs a top tether which leads to my next question:

my babysitter has a 2001 or 2002 sienna. i can find LATCH in the captain seat but can't find the tether anchor. if the car has LATCH, does it mean it also has tether anchor?

seat criteria (in decreasing order of importance):
FF 5 pt harnessed seat (LATCH or seatbelt) that can turn into HBB (harnessed) or just SPB
SIP
ease of installation in many cars (most our cars will have LATCH and top tether but may be in a passenger van too)
portable
price

I love the nautilus that I have. DS rides in it as his primary seat and I drive a lot. I have never heard a complaint from him. I think it is easy to install with both latch and a seat belt. I have it installed with a seat belt right now because he is almost at the latch limit.

The Nautilus is usually very easy to install. It has a harness, and then converts to a booster. I'd use it for the older child, and would use it for the younger as well (is that child already FF?)

don't throw rocks at me... yup. the DS2 is FF already... that was before i saw this forum. if i turned him around now, he would probably scream b/c he can't see the DVD screen. plus, i don't know how i would turn him around and still have everyone fit in our cars (i have MA/regent/SR for the pilot and MA/RN/SR for the accord). the regent and RN needs to be in the middle b/c the DS1 can climb/buckle himself in and out. since we need 2 RF and there's not enough room for the driver or front passenger. i'm welcome to suggestions if anyone has any.

anything against the safeguard go? it's on sale for $120 and it's portable so that's a big plus for me. i have to go look at the GN again and see how easy it is to maneuver. i got lazy with the britaxes... they all pretty much function the same.

any other carseats i haven't thought of?